Oxidation of alkenes is a significant reaction in both academia and industry. In line with calls for environmental protection and the concern of metal residues in products in the pharmaceutical industry, synthetic reactions under green conditions free of transition-metal catalysts are the developing trend for alkene oxidation. Selenium catalysis, an unfolding research subject, was recently involved in the topic, affording a series of methods for the synthesis of epoxides, 1,2-diols and carbonyls from alkenes. This short review aims to look back at the recent developments on selenium-catalyzed alkene oxidations at the mechanism level, and may be beneficial for designing novel selenium-containing catalysts and alkene oxidation protocols for the next phase of studies.