A20 is an anti-inflammatory protein linked to multiple human autoimmune diseases and lymphomas. A20 possesses a deubiquitinating motif and a zinc finger, ZF4, that binds ubiquitin and supports its E3 ubiquitin ligase activity. To understand how these activities mediate A20's physiological functions, we generated two lines of gene-targeted mice, abrogating either A20's deubiquitinating activity (Tnfaip3^OTU mice) or A20's ZF4 (Tnfaip3^ZF4 mice). Both Tnfaip3^OTU and Tnfaip3^ZF4 mice exhibited increased responses to TNF and sensitivity to colitis. A20's C103 deubiquitinating motif restricted both K48- and K63-linked ubiquitination of receptor interacting protein 1 (RIP1). A20's ZF4 was required for recruiting A20 to ubiquitinated RIP1. A20^OTU proteins and A20^ZF4 proteins complemented each other to regulate RIP1 ubiquitination and NFκB signaling normally in compound mutant Tnfaip3^OTU/ZF4 cells. This complementation involved homodimerization of A20 proteins, and we have defined an extensive dimerization interface in A20. These studies reveal how A20 proteins collaborate to restrict TNF signaling.