Elegant mechanisms of gene regulation have evolved to support specialised function and complexity in eukaryotes. Covalent post-translational modifications (PTMs) shape the genome and direct transcriptional programs through distinct mechanisms that operate well beyond the coding capacity of DNA. The acetylation of lysine sidechains has emerged as a particularly influential PTM of both histones and transcriptional regulators in the regulation of gene expression. Acetyllysine (AcK) marks help to organise chromatin into a state that is permissive to transcription and act as signals for the recruitment of chromatin-associated machinery and protein complexes to specific genomic loci. The messages relayed by lysine acetylation are interpreted by dedicated PTM recognition modules called bromodomains (BDs). BDs are found in many multidomain proteins that also feature unique chromatin modifying and recognising capabilities. The Bromodomain and Extraterminal domain (BET) family of BD-containing proteins have received an enormous amount of attention since their discovery, for their participation in countless transcriptional pathways and processes. Perturbation of BET function is a hallmark of several human diseases, primarily cancers and inflammation, underscoring the importance of these proteins in normal gene expression. This thesis explores several aspects of BET protein biochemistry by both examining their interactions with native targets and by characterising interactions with a new class of potential BET-BD inhibitors. Chapter 2 investigates BET family BD function in the context of modified histone recognition. Chapter 3 explores the role of the BET BDs in transcription factor (TF) recognition and function. Chapter 4 concludes the thesis by contributing to the already flourishing field of BET family BD inhibitor discovery and design through the characterisation of cyclic peptide BET BD binders.