We set out longtermist political philosophy as a research field by exploring the case for, and the implications of, ‘institutional longtermism’: the view that, when evaluating institutions, we should give significant weight to their very long-term effects. We begin by arguing that the standard case for longtermism may be more robust when applied to institutions than to individual actions or policies, both because institutions have large, broad, and long-term effects, and because institutional longtermism can plausibly sidestep various objections to individual (...) longtermism. We then address points of contact between longtermism and some central values of mainstream political philosophy, focusing on justice, equality, freedom, legitimacy, and democracy. While each value initially seems to conflict with institutional longtermism, we find that these conflicts are less clear-cut upon closer inspection, and that some political values might even provide independent support for institutional longtermism. We end with a grab bag of related questions that we lack space to explore here. (shrink)