Lysine acetyltransferases (KATs) catalyze acetylation of lysine residues on histones and other proteins to regulate chromatin dynamics and gene expression. KATs, such as CBP/p300, are under intense investigation as therapeutic targets due to their critical role in tumorigenesis of diverse cancers. The development of novel small molecule inhibitors targeting the histone acetyltransferase (HAT) function of KATs is challenging and requires robust assays that can validate the specificity and potency of potential inhibitors. This article outlines a pipeline of three methods that provide rigorous in vitro validation for novel HAT inhibitors (HATi). These methods include a test tube HAT assay, Chromatin Hyperacetylation Inhibition (ChHAI) assay, and Chromatin Immunoprecipitation-quantitative PCR (ChIP-qPCR). In the HAT assay, recombinant HATs are incubated with histones in a test tube reaction, allowing for acetylation of specific lysine residues on the histone tails. This reaction can be blocked by a HATi and the relative levels of site-specific histone acetylation can be measured via immunoblotting. Inhibitors identified in the HAT assay need to be confirmed in the cellular environment. The ChHAI assay uses immunoblotting to screen for novel HATi that attenuate the robust hyperacetylation of histones induced by a histone deacetylase inhibitor (HDACi). The addition of an HDACi is helpful because basal levels of histone acetylation can be difficult to detect via immunoblotting. The HAT and ChHAI assays measure global changes in histone acetylation, but do not provide information regarding acetylation at specific genomic regions. Therefore, ChIP-qPCR is used to investigate the effects of HATi on histone acetylation levels at gene regulatory elements. This is accomplished through selective immunoprecipitation of histone-DNA complexes and analysis of the purified DNA through qPCR. Together, these three assays allow for the careful validation of the specificity, potency, and mechanism of action of novel HATi.