Caspase recruitment domain (CARD)-containing protein 9 (CARD9) and CARD-containing protein 11 (CARD11, also known as CARMA1) are paralogous proteins that act in a conserved manner as scaffolding proteins required to propagate signaling in immune cells. CARD9 functions in myeloid cells during innate immune responses to activate NF-kB and p38 MAPK. CARD11 acts in lymphoid cells downstream of T-cell receptors and B-cell receptors and also leads to activation of NF-kB. In these cell types, CARD11 is critical for proper cellular activation and proliferation upon receptor engagement. CARD9 and CARD11 mediate signal transduction in immune cells by nucleating Bcl10 polymerization, but are held in an autoinhibited state prior to activation. We have elucidated the structural basis for this autoinhibition by determining the structure of a region of CARD9 that includes an extensive interface between its caspase recruitment domain (CARD) and coiled-coil domain. We demonstrate, for both CARD9 and CARD11, that disruption of this interface leads to hyperactivation in cells and to the formation of Bcl10-templating filaments in vitro, thereby illuminating the mechanism of action of a number of oncogenic mutations of CARD11. These structural insights enable us to characterize two similar, yet distinct, mechanisms by which autoinhibition is relieved in the course of canonical CARD9 or CARD11 activation. We have also solved the structure of the CARD9 filament and dissect the molecular determinants of helical template assembly. Taken together, these findings delineate the inhibitory and activation mechanisms within the protein family.