Histone deacetylases (HDACs) are enzymes that remove acetyl groups from lysine residues of histones and nonhistone proteins. In humans, there are 18 HDAC enzymes, divided into four classes: Class I (HDAC1, HDAC2, HDAC3, HDAC8), Class II (HDAC4, HDAC5, HDAC6, HDAC7, HDAC9, HDAC10), Class III (SIRT1, SIRT2, SIRT3, SIRT4, SIRT5, SIRT6, SIRT7), and Class IV (HDAC11). HDACs regulate chromatin structure and transcription by deacetylating histones, and also control diverse cellular processes by deacetylating nonhistones. HDAC inhibitors are potential anticancer agents and show promise for treating many diseases. HDACs are regulated by various mechanisms, including protein-protein interactions and posttranslational modifications. They are involved in many biological processes, including gene transcription, chromatin structure, and epigenetic regulation. HDACs are also involved in the regulation of nonhistone proteins, such as p53 and α-tubulin. HDACs are found in various subcellular locations, including the nucleus, cytoplasm, and mitochondria. The activity of HDACs is regulated by phosphorylation, which can activate or repress their enzymatic activity. HDACs are also regulated by protein complexes, such as the Sin3, NuRD, and CoREST complexes. The regulation of HDAC activity is important for understanding their role in human health and disease. HDAC inhibitors are being developed as potential therapeutic agents for various diseases, including cancer.Histone deacetylases (HDACs) are enzymes that remove acetyl groups from lysine residues of histones and nonhistone proteins. In humans, there are 18 HDAC enzymes, divided into four classes: Class I (HDAC1, HDAC2, HDAC3, HDAC8), Class II (HDAC4, HDAC5, HDAC6, HDAC7, HDAC9, HDAC10), Class III (SIRT1, SIRT2, SIRT3, SIRT4, SIRT5, SIRT6, SIRT7), and Class IV (HDAC11). HDACs regulate chromatin structure and transcription by deacetylating histones, and also control diverse cellular processes by deacetylating nonhistones. HDAC inhibitors are potential anticancer agents and show promise for treating many diseases. HDACs are regulated by various mechanisms, including protein-protein interactions and posttranslational modifications. They are involved in many biological processes, including gene transcription, chromatin structure, and epigenetic regulation. HDACs are also involved in the regulation of nonhistone proteins, such as p53 and α-tubulin. HDACs are found in various subcellular locations, including the nucleus, cytoplasm, and mitochondria. The activity of HDACs is regulated by phosphorylation, which can activate or repress their enzymatic activity. HDACs are also regulated by protein complexes, such as the Sin3, NuRD, and CoREST complexes. The regulation of HDAC activity is important for understanding their role in human health and disease. HDAC inhibitors are being developed as potential therapeutic agents for various diseases, including cancer.