1. We present an R-package, stagePop, which is a tool for predicting the deterministic dynamics and interactions of stage-structured populations (i.e. where the life cycle consists of distinct stages, for example eggs, juveniles and reproductive adults). 2. The continuous-time formulation enables stagePop to easily simulate time-varying stage durations, overlapping generations and density-dependent vital rates. 3. The package can be used to predict predator-prey interactions, host-parasitoid interactions, resource competition, intra-specific competition and the effects of environmental change on stage-structured (and non-stage structured) species. 4. It can be used for predicting the effects of bio-control and climate change, investigating mechanisms for maintaining diversity, predicting the dynamics of complex food webs following perturbation and so on. 5. The code is based on the formulation by Nisbet and Gurney (Theoretical Population Biology, 23, 1983, 114) using delay-differential equations, which are solved using the R-packages deSolve or PBSddesolve.