This paper analyzes how endogenous imperfect exchange rate pass-through a¤ects inflation targeting optimal monetary policies in a New Keynesian small open economy. The paper shows that an inverse relation exists between the pass- through and the insulation of the economy from foreign and monetary policy shocks, and that imperfect pass-through tends to decrease the variability of the terms of trade. Furthermore, with CPI inflation targeting, in the short run, delayed pass-through constrains monetary policy more than incomplete pass- through and interest rate smoothing amplifies this e¤ect. When the pass-through decreases, the variability in economic activity tends to raise and the trade-o¤ between the stabilization of CPI inflation and output worsens in direct relation to how strictly the central bank is targeting CPI inflation. In contrast, with domestic inflation targeting, optimal monetary policy is not constrained and opposite results occur. Consequently, imperfect pass-through favors the choice of domestic to CPI inflation targeting.