As sequence and structural databases grow along with powerful analysis tools, the prevalence and diversity of pseudoenzymes have become increasingly evident. Pseudoenzymes are present across the tree of life in a large number of enzyme families. Pseudoenzymes are defined as proteins that lack conserved catalytic motifs based on sequence analysis. However, some pseudoenzymes may have migrated amino acids necessary for catalysis, allowing them to catalyze enzymatic reactions. Furthermore, pseudoenzymes retain several non-enzymatic functions such as allosteric regulation, signal integration, scaffolding, and competitive inhibition. In this review, we provide examples of each mode of action using the pseudokinase, pseudophosphatase, and pseudo ADP-ribosyltransferase families. We highlight the methodologies that facilitate the biochemical and functional characterization of pseudoenzymes to encourage further investigation in this burgeoning field.