The Julia-Kocienski olefination reaction has become over past 30 years since its discovery one of the key C-C connective methods that is used in the late-stage natural product synthesis. The reaction proceeds under mild reaction conditions, with wide substrate scope and functional group tolerance, and with high (E) selectivity. In this review, we discuss the reaction from a mechanistic point of view and disclose key features that play an important role in reaction selectivity. Finally, the mechanistic aspects of the newly developed modification of the Julia-Kocienski reaction, which allows the formation of both (E) and (Z) olefins, from the same reaction partners, are discussed.