This review is intended to highlight the recent developments and trends in the chemistry of oxime esters-based photoinitiators. It focuses on the mechanisms underlying the polymerization processes, measurement techniques, and various factors that affect the conversion efficiency. Due to their remarkable reactivities in photopolymerization reactions, oxime esters have attracted a great attention as reliable Type I (one-component) photoinitiators. In recent years, several research studies have been published with the purpose of modifying their chemical structures by introducing different chromophores or by varying the substitution pattern of the ester group, for which an interesting structure/reactivity/efficiency relationship could be achieved. In this review, an overview of the recent advances on oxime esters-based photoinitiators, as well as a comparison of the various scaffolds proposed in the literature is provided and discussed.