Drawing is the starting point for many kinds of tasks, for everyone from children making pictures to professional architects sketching ideas. Drawing seems to be fundamentally connected to how we represent the world visually. Most computer graphics focuses on realistic visual simulation, but over the past few decades, line drawing algorithms have matured, providing the ability to automatically create reasonable line drawings from 3D geometry. This tutorial provides a detailed guide to the mathematical theory and computer algorithms for line drawing of 3D objects. It focuses on the curves known as contours as they are the most important curves for line drawing of 3D surfaces. The authors describe the different algorithms required to compute and render these curves, before going on to explain boundary curves and surface-surface intersection curves. The tutorial concludes with other topics in 3D non-photorealistic rendering including: other types of curves, stroke rendering, and non-ph