This invention provides novel caspase inhibitors of formula I: wherein R1 is hydrogen, CHN2, R, or —CH2Y; R is an aliphatic group, an aryl group, an aralkyl group, a heterocyclyl group, or a heterocyclylalkyl group; Y is an electronegative leaving group; R2 is CO2H, CH2CO2H, or esters, amides or isosteres thereof; X2—X1 is N(R3)—C(R3), C(R3)2—C(R3), C(R3)2—N, N═C, C(R3)═C, C(═O)—N, or C(═O)—C(R3); each R3 is independently selected from hydrogen or C1-6 aliphatic; Ring C is a fused aryl ring; n is 0, 1 or 2; and each methylene carbon in Ring A is optionally and independently substituted by ═O, or one or more halogen, C1-4 alkyl, or C1-4 alkoxy. The compounds are useful for treating caspase-mediated diseases.