The highly reactive H3+ molecular ion plays a central and critical role in the chain of interstellar chemistry beyond that touched upon in this Chapter. The Figure of reaction steps to the right by Ben McCall illustrates how it is a vital step in the pathway to carbon compounds. McCall’s thesis is an excellent resource for learning more. Until recently there was considerable discrepancy between the theoretical predictions for the reaction cross sections leading to H3+ and the experimental determination. There remains considerable discussion about the actual production mechanisms of H3+ in molecular clouds.