SENPs are proteases that participate in the regulation of SUMOylation by generating mature small ubiquitin-related modifiers (SUMO) for protein conjugation (endopeptidase activity) and removing conjugated SUMO from targets (isopeptidase activity). Using purified recombinant catalytic domains of 6 of the 7 human SENPs, we demonstrate the specificity of their respective activities on SUMO-1, -2, and -3. The primary mode of recognition of substrates is via the SUMO domain, and the C-terminal tails direct endopeptidase specificity. Broadly speaking, SENP1 is the most efficient endopeptidase, whereas SENP2 and -5-7 have substantially higher isopeptidase than endopeptidase activities. We developed fluorogenic tetrapeptide substrates that are cleaved by SENPs, enabling us to characterize the environmental profiles of each enzyme. Using these synthetic substrates we reveal that the SUMO domain enhances catalysis of SENP1, -2, -5, -6, and -7, demonstrating substrate-induced activation of SENPs by SUMOs.