In contrast to mechanism-based (or suicide) inhibitors, covalent inhibitors do not require enzyme activation and (in many cases) form bonds to noncatalytic residues of their targets. Covalent inhibitors can be divided into reversible and irreversible covalent inhibitors based on the mechanism and kinetics of inhibition. In reversible covalent inhibitors, protein activity can be recovered via the cleavage of the formed bond; they are continually binding and unbinding with their target. The rates of dissociation for irreversible covalent inhibitors are lower than the rates of resynthesis of their targets unlike reversible covalent inhibitors which undergo covalent bond cleavage with rates greater than the rates of target resynthesis. Thus, irreversible covalent inhibitors form a permanent covalent bond with their target. The duration of effect of covalent inhibitors depends both on the type of covalent inhibitor and on the rate of resynthesis of their targets.