Functional amino acids that play critical roles in catalysis and regulation are known to display elevated nucleophilicity and can be selectively targeted for covalent modification by reactive electrophiles. Chemical-proteomic platforms, such as activity-based protein profiling (ABPP), exploit this reactivity by utilizing chemical probes to covalently modify active-site residues to inform on the functional state of enzymes within complex proteomes. These and other applications rely on the availability of a diverse array of electrophiles and detailed knowledge of the reactivity and amino-acid selectivity of these groups. Here, we survey the current landscape of electrophiles that covalently target various nucleophilic amino acids in proteins and highlight proteomic applications that have benefited from the unique properties of these electrophiles.