diff --git a/src/investment/portfolio/Portfolio.exception.ts b/src/investment/portfolio/Portfolio.exception.ts
new file mode 100644
index 0000000..81abd28
--- /dev/null
+++ b/src/investment/portfolio/Portfolio.exception.ts
@@ -0,0 +1,23 @@
+import { HttpException, HttpStatus } from '@nestjs/common';
+
+/**
+ * Base class for all portfolio-domain exceptions.
+ * Every derived class must supply a machine-readable `errorCode` so API
+ * consumers can handle specific failure modes without parsing error messages.
+ */
+export abstract class PortfolioException extends HttpException {
+  readonly errorCode: string;
+
+  constructor(message: string, errorCode: string, status: HttpStatus) {
+    super(
+      {
+        statusCode: status,
+        errorCode,
+        message,
+        domain: 'portfolio',
+      },
+      status,
+    );
+    this.errorCode = errorCode;
+  }
+}
\ No newline at end of file
diff --git a/src/investment/portfolio/dto/create-portfolio.dto.ts b/src/investment/portfolio/dto/create-portfolio.dto.ts
new file mode 100644
index 0000000..a816eae
--- /dev/null
+++ b/src/investment/portfolio/dto/create-portfolio.dto.ts
@@ -0,0 +1,63 @@
+import {
+    IsString,
+    IsNotEmpty,
+    IsOptional,
+    IsEnum,
+    IsNumber,
+    Min,
+    Max,
+    MaxLength,
+    IsArray,
+    ArrayMinSize,
+    ArrayMaxSize,
+  } from 'class-validator';
+  import { Type } from 'class-transformer';
+  
+  export enum RiskTolerance {
+    CONSERVATIVE = 'conservative',
+    MODERATE = 'moderate',
+    AGGRESSIVE = 'aggressive',
+  }
+  
+  export enum RebalancingFrequency {
+    DAILY = 'daily',
+    WEEKLY = 'weekly',
+    MONTHLY = 'monthly',
+    QUARTERLY = 'quarterly',
+    MANUAL = 'manual',
+  }
+  
+  export class CreatePortfolioDto {
+    @IsString()
+    @IsNotEmpty({ message: 'Portfolio name is required.' })
+    @MaxLength(100, { message: 'Portfolio name must not exceed 100 characters.' })
+    name: string;
+  
+    @IsOptional()
+    @IsString()
+    @MaxLength(500, { message: 'Description must not exceed 500 characters.' })
+    description?: string;
+  
+    @IsEnum(RiskTolerance, {
+      message: `riskTolerance must be one of: ${Object.values(RiskTolerance).join(', ')}.`,
+    })
+    riskTolerance: RiskTolerance;
+  
+    @IsNumber({}, { message: 'initialBalance must be a number.' })
+    @Min(0.01, { message: 'initialBalance must be greater than 0.' })
+    @Type(() => Number)
+    initialBalance: number;
+  
+    @IsOptional()
+    @IsEnum(RebalancingFrequency, {
+      message: `rebalancingFrequency must be one of: ${Object.values(RebalancingFrequency).join(', ')}.`,
+    })
+    rebalancingFrequency?: RebalancingFrequency;
+  
+    @IsOptional()
+    @IsArray()
+    @ArrayMinSize(1, { message: 'At least one target asset must be provided.' })
+    @ArrayMaxSize(50, { message: 'A portfolio may not exceed 50 target assets.' })
+    @IsString({ each: true })
+    targetAssetIds?: string[];
+  }
\ No newline at end of file
diff --git a/src/investment/portfolio/index.ts b/src/investment/portfolio/index.ts
new file mode 100644
index 0000000..3ef40ef
--- /dev/null
+++ b/src/investment/portfolio/index.ts
@@ -0,0 +1,7 @@
+export { PortfolioException } from './Portfolio.exception';
+export { InsufficientBalanceException } from './insufficient-balance.exception';
+export { OptimizationFailedException } from './optimization-failed.exception';
+export type { OptimizationFailureReason } from './optimization-failed.exception';
+export { RebalancingFailedException } from './rebalancing-failed.exception';
+export type { RebalancingFailureReason } from './rebalancing-failed.exception';
+export { PortfolioNotFoundException } from './Portfolio-not-found.exception';
\ No newline at end of file
diff --git a/src/investment/portfolio/insufficient-balance.exception.ts b/src/investment/portfolio/insufficient-balance.exception.ts
new file mode 100644
index 0000000..1588a74
--- /dev/null
+++ b/src/investment/portfolio/insufficient-balance.exception.ts
@@ -0,0 +1,32 @@
+import { HttpStatus } from '@nestjs/common';
+import { PortfolioException } from './Portfolio.exception';
+
+/**
+ * Thrown when a financial operation (trade, rebalance, withdrawal) cannot
+ * proceed because the portfolio lacks sufficient balance.
+ *
+ * @example
+ * throw new InsufficientBalanceException({
+ *   required: 5000,
+ *   available: 3200,
+ *   currency: 'USD',
+ * });
+ */
+export class InsufficientBalanceException extends PortfolioException {
+  readonly required: number;
+  readonly available: number;
+  readonly currency: string;
+
+  constructor(params: { required: number; available: number; currency?: string }) {
+    const { required, available, currency = 'USD' } = params;
+    super(
+      `Insufficient balance: operation requires ${currency} ${required.toFixed(2)} ` +
+        `but only ${currency} ${available.toFixed(2)} is available.`,
+      'PORTFOLIO_INSUFFICIENT_BALANCE',
+      HttpStatus.UNPROCESSABLE_ENTITY,
+    );
+    this.required = required;
+    this.available = available;
+    this.currency = currency;
+  }
+}
\ No newline at end of file
diff --git a/src/investment/portfolio/optimization-failed.exception.ts b/src/investment/portfolio/optimization-failed.exception.ts
new file mode 100644
index 0000000..83aed61
--- /dev/null
+++ b/src/investment/portfolio/optimization-failed.exception.ts
@@ -0,0 +1,52 @@
+import { HttpStatus } from '@nestjs/common';
+import { PortfolioException } from './Portfolio.exception';
+
+export type OptimizationFailureReason =
+  | 'CONVERGENCE_FAILURE'      // Solver did not converge within iteration limit
+  | 'INFEASIBLE_CONSTRAINTS'   // Constraints are mutually exclusive
+  | 'INSUFFICIENT_ASSETS'      // Not enough eligible assets to optimise
+  | 'MODEL_UNAVAILABLE'        // ML model endpoint unreachable / timed out
+  | 'INVALID_RISK_PARAMETERS'  // Risk tolerance input outside supported range
+  | 'UNKNOWN';
+
+/**
+ * Thrown when the portfolio optimisation engine (mean-variance, ML-based, etc.)
+ * cannot produce a valid allocation for the given inputs.
+ *
+ * @example
+ * throw new OptimizationFailedException(
+ *   'CONVERGENCE_FAILURE',
+ *   { iterations: 1000, tolerance: 1e-6 },
+ * );
+ */
+export class OptimizationFailedException extends PortfolioException {
+  readonly reason: OptimizationFailureReason;
+  readonly context?: Record<string, unknown>;
+
+  constructor(
+    reason: OptimizationFailureReason = 'UNKNOWN',
+    context?: Record<string, unknown>,
+  ) {
+    const reasonMessages: Record<OptimizationFailureReason, string> = {
+      CONVERGENCE_FAILURE:
+        'Portfolio optimisation did not converge. Try relaxing constraints or increasing the iteration limit.',
+      INFEASIBLE_CONSTRAINTS:
+        'The provided constraints cannot be satisfied simultaneously. Review allocation bounds.',
+      INSUFFICIENT_ASSETS:
+        'Not enough eligible assets are available to run optimisation. A minimum of 2 assets is required.',
+      MODEL_UNAVAILABLE:
+        'The ML optimisation model is currently unavailable. Please retry shortly.',
+      INVALID_RISK_PARAMETERS:
+        'Risk tolerance parameters are outside the supported range [0, 1].',
+      UNKNOWN: 'Portfolio optimisation failed for an unknown reason.',
+    };
+
+    super(
+      reasonMessages[reason],
+      'PORTFOLIO_OPTIMIZATION_FAILED',
+      HttpStatus.UNPROCESSABLE_ENTITY,
+    );
+    this.reason = reason;
+    this.context = context;
+  }
+}
\ No newline at end of file
diff --git a/src/investment/portfolio/portfolio-exception.filter.ts b/src/investment/portfolio/portfolio-exception.filter.ts
new file mode 100644
index 0000000..c464ebd
--- /dev/null
+++ b/src/investment/portfolio/portfolio-exception.filter.ts
@@ -0,0 +1,44 @@
+import {
+    ArgumentsHost,
+    Catch,
+    ExceptionFilter,
+    HttpStatus,
+    Logger,
+  } from '@nestjs/common';
+  import { Request, Response } from 'express';
+import { PortfolioException } from './Portfolio.exception';
+  
+  /**
+   * Catches any `PortfolioException` (and subclasses) and serialises it into a
+   * consistent structured JSON envelope.
+   *
+   * Register globally in `main.ts`:
+   * ```ts
+   * app.useGlobalFilters(new PortfolioExceptionFilter());
+   * ```
+   * Or per-controller via `@UseFilters(PortfolioExceptionFilter)`.
+   */
+  @Catch(PortfolioException)
+  export class PortfolioExceptionFilter implements ExceptionFilter {
+    private readonly logger = new Logger(PortfolioExceptionFilter.name);
+  
+    catch(exception: PortfolioException, host: ArgumentsHost): void {
+      const ctx = host.switchToHttp();
+      const response = ctx.getResponse<Response>();
+      const request = ctx.getRequest<Request>();
+  
+      const status = exception.getStatus() ?? HttpStatus.INTERNAL_SERVER_ERROR;
+      const body = exception.getResponse() as Record<string, unknown>;
+  
+      this.logger.warn(
+        `[PortfolioException] ${body['errorCode']} — ${body['message']} | ` +
+          `${request.method} ${request.url}`,
+      );
+  
+      response.status(status).json({
+        ...body,
+        timestamp: new Date().toISOString(),
+        path: request.url,
+      });
+    }
+  }
\ No newline at end of file
diff --git a/src/investment/portfolio/portfolio-not-found.exception.ts b/src/investment/portfolio/portfolio-not-found.exception.ts
new file mode 100644
index 0000000..28936c2
--- /dev/null
+++ b/src/investment/portfolio/portfolio-not-found.exception.ts
@@ -0,0 +1,16 @@
+import { HttpStatus } from '@nestjs/common';
+import { PortfolioException } from './Portfolio.exception';
+
+/**
+ * Thrown when a portfolio lookup by ID yields no result.
+ * Keeps 404 semantics domain-specific so filters can handle it precisely.
+ */
+export class PortfolioNotFoundException extends PortfolioException {
+  constructor(portfolioId: string) {
+    super(
+      `Portfolio with id "${portfolioId}" was not found.`,
+      'PORTFOLIO_NOT_FOUND',
+      HttpStatus.NOT_FOUND,
+    );
+  }
+}
\ No newline at end of file
diff --git a/src/investment/portfolio/rebalancing-failed.exception.ts b/src/investment/portfolio/rebalancing-failed.exception.ts
new file mode 100644
index 0000000..dd3cf53
--- /dev/null
+++ b/src/investment/portfolio/rebalancing-failed.exception.ts
@@ -0,0 +1,50 @@
+import { HttpStatus } from '@nestjs/common';
+import { PortfolioException } from './Portfolio.exception';
+
+export type RebalancingFailureReason =
+  | 'MARKET_CLOSED'            // Cannot trade outside market hours
+  | 'PRICE_STALENESS'          // Asset prices are too stale to rebalance safely
+  | 'ALLOCATION_DRIFT_SAFE'    // Drift is within tolerance; rebalance not needed
+  | 'PARTIAL_EXECUTION'        // Some legs of the rebalance failed to execute
+  | 'LOCK_CONFLICT'            // Another rebalance is already in progress
+  | 'UNKNOWN';
+
+/**
+ * Thrown when a portfolio rebalancing operation cannot be completed.
+ * Distinct from `OptimizationFailedException` — this covers execution-time
+ * failures rather than planning-time failures.
+ *
+ * @example
+ * throw new RebalancingFailedException('MARKET_CLOSED', { openAt: '09:30 EST' });
+ */
+export class RebalancingFailedException extends PortfolioException {
+  readonly reason: RebalancingFailureReason;
+  readonly context?: Record<string, unknown>;
+
+  constructor(
+    reason: RebalancingFailureReason = 'UNKNOWN',
+    context?: Record<string, unknown>,
+  ) {
+    const reasonMessages: Record<RebalancingFailureReason, string> = {
+      MARKET_CLOSED:
+        'Rebalancing cannot proceed while the market is closed.',
+      PRICE_STALENESS:
+        'Asset prices are too stale to execute a safe rebalance. Prices will be refreshed automatically.',
+      ALLOCATION_DRIFT_SAFE:
+        'Portfolio allocation drift is within tolerance thresholds. No rebalancing is required.',
+      PARTIAL_EXECUTION:
+        'Rebalancing partially executed. Some trade legs failed. Manual review is required.',
+      LOCK_CONFLICT:
+        'A rebalancing operation is already in progress for this portfolio. Please wait.',
+      UNKNOWN: 'Rebalancing failed for an unknown reason.',
+    };
+
+    super(
+      reasonMessages[reason],
+      'PORTFOLIO_REBALANCING_FAILED',
+      HttpStatus.UNPROCESSABLE_ENTITY,
+    );
+    this.reason = reason;
+    this.context = context;
+  }
+}
\ No newline at end of file