Histone deacetylase inhibitors (HDIs) are good anti-cancer targets. Histone acetyltransferases (HATs) and HDIs play opposite roles in the regulation of gene expression via an epigenetic mechanism. HATs catalyze the acetylation of lysine residues in histone tails, facilitating and sustaining gene transcription. HDIs remove the acetyl moieties from the epsilon-amine of lysine residues of histone tails, leading to a more condensed form of chromatin and preventing gene transcription. This book focuses on the use of HDIs for the treatment of lymphoid malignancies; myeloid malignancies; and breast cancer.