In recent years, a number of bioorthogonal reactions have been developed, exemplified by click chemistry, that enable the efficient formation of a specific product, even within a highly complex chemical environment. While the exquisite selectivity and reliability of these transformations have led to their broad application in diverse research areas, they have proven to be particularly beneficial to biological studies. In this regard, the ability to rationally incorporate reactive tags onto a biomolecular target and subsequently achieve high selectivity in tag derivatization within a complex biological sample has revolutionized the toolbox that is available for addressing fundamental issues. Herein, an introduction to the impact of click chemistry and other bioorthogonal reactions on the study of biological systems is presented. This includes discussion of the philosophy behind click chemistry, the details and benefits of bioorthogonal reactions that have been developed, and examples of recent innovative approaches that have effectively exploited these transformations to study cellular processes. For the latter, the impacts of bioorthogonal reactions on drug design (i.e., in situ combinatorial drug design), biomolecule labeling and detection (site-specific derivatization of proteins, viruses, sugars, DNA, RNA, and lipids), and the recent strategy of activity-based protein profiling are highlighted.