Bacteria can receive genetic material from other bacteria or invading bacteriophages primarily through horizontal gene transfer. These genetic exchanges can result in genome rearrangement and the acquisition of novel traits that assist cells with stresses and adverse environmental conditions. Bacteria have a relatively small genome with >90% of sequences consisting of protein coding genes, stable RNA biomolecules, and gene regulatory sequences. The remaining genome fraction is primarily large repeat elements, such as retrotransposons, interspersed repeat elements, insertion sequences, and the more recently discovered clustered regularly interspaced short palindromic repeats (CRISPRs), with CRISPR-associated gene sequences (cas) that code for various Cas proteins. The CRISPR genetic locus is a series of direct repeats that are interspersed by unique spacer sequences. These unique spacer sequences represent signatures of bacteriophage genomes as the "working memory" for a bacterium to identify and destroy an invading phage genome that has previously infected the host. The protective function of the CRISPR-Cas systems are found in ∼40% of sequenced bacterial genomes, and it is often defined as bacterial acquired immunity. This chapter will elaborate the origin, structure, and function of CRISPR-Cas genetic systems acquired by bacteria, and their role in adaptive fitness while being subjected to environmental stress conditions.