Sydnonimines are intriguing compounds belonging to the mesoionic family. To date, only a limited number of research groups have studied their chemistry and use in organic synthesis, medicinal chemistry and chemical biology. This review aims at providing an overview of the synthesis and the properties of sydnonimines and the most recent developments in their use as tools for chemical biology. The recent discovery that sydnonimines can act as a dipole to undergo bioorthogonal click-and-release reactions with cycloalkynes has stimulated a renewed interest from the scientific community. Given the high potential of these mesoionics, we believe that major developments are to be expected in the field of bioorthogonal chemistry.