This volume opens by covering two main types of approaches widely used to determine essential genes: single-gene knockouts and transposon mutagenesis, in both prokaryotes and Candida albicans. Given the significant advancement in the computational predictions of microbial essential genes, the second half of the book examines four main types of approaches: comparative genomics, supervised machine learning, constraint-based methods, and corrections of transposon mutagenesis data, as well as databases and servers that are often used in studying gene essentiality. Written in the highly successful Methods in Molecular Biology series format, chapters include an introduction to their respective topics, lists of the necessary materials and reagents, step-by- step,