The Gilbert-Johnson-Keerthi (GJK) algorithm is an efficient method for computing the distance between two convex shapes. It has become a popular method for real-time 3D collision detection. While less commonly used for 2D collision detection, the GJK algorithm still maintains several of its advantages. Its biggest disadvantage is that the algorithm can be conceptually difficult to understand (although it is easy to implement). This tutorial will explain how the GJK algorithm works in detail and demonstrate how to apply it to 2D collision detection.