Mistakes are a window into students' mathematical understanding. Taking the time to analyze mistakes can inform you of where students are in their thinking. They can also guide you with respect to how to proceed with instruction. Introduced here are two categories of mistakes: meaty mistakes and minor mistakes. Meaty mistakes demonstrate gaps in understanding and take time to reteach. Minor mistakes are errors in accuracy and can be addressed by assisting students in developing methods to check their work. This article briefly describes both types of mistakes and how to address them.