For the first 30 years since its discovery, reversible protein acetylation has been studied and understood almost exclusively in the context of histone modification and gene transcription. With the discovery of non–histone acetylated proteins and acetylation-modifying enzymes in cellular compartments outside the nucleus, the regulatory potential of reversible acetylation has slowly been recognized in the last decade. However, the scope of protein acetylation involvement in complex biological processes remains uncertain. The recent development of new technology has enabled, for the first time, the identification and quantification of the acetylome, acetylation events at the whole-proteome level. These efforts have uncovered a stunning complexity of the acetylome that potentially rivals that of the phosphoproteome. The remarkably ubiquitous and conserved nature of protein acetylation revealed by these new studies suggests the regulatory power of this dynamic modification. The establishment of comprehensive acetylomes will change the landscape of protein acetylation, where an exciting research frontier awaits.