Dipeptide nitrile Cathepsin K inhibitors of formula I, and pharmaceutically acceptable salts or esters thereof In which R.sub.1 and R.sub.2 are independently H or C.sub.1-C.sub.7lower alkyl, or R.sub.1 and R.sub.2 together with the carbon atom to which they are attached form a C.sub.3-C.sub.8cycloalkyl ring, and Het is an optionally substituted nitrogen-containing heterocyclic substituent, are provided, useful e.g. for therapeutic or prophylactic treatment of a disease or medical condition in which cathepsin K is implicated.