The chapter discusses the role of histone acetyltransferases (HATs) in transcriptional regulation, focusing on the mechanisms and substrates of these enzymes. HATs, which belong to the GNAT superfamily, are crucial for modifying histones and other proteins, thereby influencing gene expression. The chapter highlights the structure and mechanism of action of HATs, particularly the yeast Gcn5 and its human homologs GCN5 and PCAF. These enzymes acetylate lysine residues on histone tails, neutralizing their positive charge and weakening interactions with DNA or other histones, thus facilitating transcriptional activation. The chapter also covers other HATs, such as Hat1, Elp3, Hpa2, and members of the MYST family, which play roles in various cellular processes including transcriptional silencing and dosage compensation. Additionally, it discusses the multifunctional nature of HATs like p300/CBP, which can act as both HATs and factor acetyltransferases (FATs), and their interactions with transcription factors and coactivators. The chapter concludes by emphasizing the reversible nature of histone acetylation and the importance of deacetylases in maintaining appropriate levels of transcription.The chapter discusses the role of histone acetyltransferases (HATs) in transcriptional regulation, focusing on the mechanisms and substrates of these enzymes. HATs, which belong to the GNAT superfamily, are crucial for modifying histones and other proteins, thereby influencing gene expression. The chapter highlights the structure and mechanism of action of HATs, particularly the yeast Gcn5 and its human homologs GCN5 and PCAF. These enzymes acetylate lysine residues on histone tails, neutralizing their positive charge and weakening interactions with DNA or other histones, thus facilitating transcriptional activation. The chapter also covers other HATs, such as Hat1, Elp3, Hpa2, and members of the MYST family, which play roles in various cellular processes including transcriptional silencing and dosage compensation. Additionally, it discusses the multifunctional nature of HATs like p300/CBP, which can act as both HATs and factor acetyltransferases (FATs), and their interactions with transcription factors and coactivators. The chapter concludes by emphasizing the reversible nature of histone acetylation and the importance of deacetylases in maintaining appropriate levels of transcription.