This document discusses interaction design and its importance. Interaction design is the art of defining how users interact with products and systems. It focuses on making products useful, usable, engaging and fun for users. Good interaction design facilitates richer interactions between people and products. The document outlines common challenges in interaction design like lack of budget and time. It provides examples of good and bad design and discusses key roles in interaction design projects like research, design, implementation and usability testing. A case study example shows how subtly changing a checkout process reduced sales by breaking the expected interaction flow.