This document introduces aspect-oriented programming (AOP) and how it can be used to separate cross-cutting concerns from core business logic. It discusses how AOP frameworks like PostSharp allow aspects to modularize features like logging, transactions, and security. The document provides an example of how a rental booking method grows over time to include these concerns. It compares different AOP frameworks and concludes that PostSharp allows building aspects with strong robustness while minimizing code invasiveness.