The translocon at the inner envelope membrane of chloroplasts (Tic) plays a central role in plastid biogenesis by coordinating the sorting of nucleus-encoded preproteins across the inner membrane and coordinating the interactions of preproteins with the processing and folding machineries of the stroma. Despite these activities, the precise roles of known Tic proteins in translocation, sorting, and preprotein maturation have not been defined. In this report, we examine the in vivo function of a major Tic component, Tic110. We demonstrate that Arabidopsis thaliana Tic110 (atTic110) is essential for plastid biogenesis and plant viability. The downregulation of atTic110 expression results in the reduced accumulation of a wide variety of plastid proteins. The expression of dominant negative mutants of atTic110 disrupts assembly of Tic complexes and the translocation of preproteins across the inner envelope membrane. Together, these data suggest that Tic110 plays a general role in the import of nuclear-encoded preproteins as a common component of Tic complexes.