In the first section of this introduction, we provide background information for yeast two-hybrid (Y2H) assays that provide a genetic method for the identification and analysis of binary protein-protein interactions and that are complementary to biochemical methods such as immunoprecipitation. In the second section, we discuss yeast one-hybrid (Y1H) assays that provide a "gene-centered" (DNA-to-protein) genetic method to identify and study protein-DNA interactions between cis-regulatory elements and transcription factors (TFs). This method is complementary to "TF-centered" (protein-to-DNA) biochemical methods such as chromatin immunoprecipitation.