QCD at fixed topology

Since present Monte Carlo algorithms for lattice QCD may become trapped in a fixed topological charge sector, it is important to understand the effect of calculating at fixed topology. In this Letter, we show that although the restriction to a fixed topological sector becomes irrelevant in the infinite volume limit, it gives rise to characteristic finite-size effects due to contributions from all θ-vacua. We calculate these effects and show how to extract physical results from numerical data obtained at fixed topology.