This book provides state-of-the-art information on gene essentiality screenings in a wide variety of organisms, i.e. screening for protein-coding genes and other genomic elements that are required by an organism to survive under specific conditions. With a focus on the two techniques that have revolutionized the field, the collection begins with chapters employing CRISPR/Cas9-based approaches followed by Tn-seq-based approaches, but later chapters also delve into other techniques for exploring essential genes, such as bioinformatics methods. Written for the highly successful Methods in Molecular Biology series, chapters include introductions to their respective topics, lists of the necessary materials and reagents, step-by-step, readily reproducible laboratory protocols, and tips on troubleshooting and avoiding known pitfalls. Comprehensive and authoritative, Essential Genes and Genomes: Methods and Protocols is an ideal guide for researchers attempting to strip genetics down to its fundamentals.