This document discusses paper prototyping and concept testing. It provides an overview of prototyping, including that prototypes are sample versions that communicate ideas visually and allow testing without significant investment. Different levels of prototyping are described from low-fidelity paper prototypes to high-fidelity digital versions. The benefits of prototyping like gathering feedback and validating ideas are outlined. A technique called "Crazy 8s" is introduced where ideas are sketched for one minute each over 8 minutes to generate concepts. Examples of paper prototyping for apps are shown and tips for getting started like drawing circles and boxes are provided.