Mendelian mutations are the most medically actionable variants in the human genome and have always played a central role in its functional annotation. Despite the relative ease with which Mendelian mutations are identified compared to other classes of variants, the pace of their discovery has until recently been slow. However, recent technological advances in genomic sequencing have made the prospect of identifying all genes that can harbor Mendelian mutations an achievable near-term goal. The many lessons learned from previous discoveries of Mendelian mutations should inform future studies as I will discuss in this review. Also discussed are some of the challenges that will gain more prominence as we approach the last phase of the effort to map all Mendelian genes.