A large-scale, cross-taxa analysis reveals high nonlinearity and limited long-term predictability in the dynamics of animal populations Dynamics of animal populations fluctuate in a variety of ways. They may periodically oscillate, wax and wane irregularly, boom-and-bust, or alternate between periods with different types of fluctuations. All these complex patterns are driven by a combination of biotic and abiotic factors, either stochastic or deterministic 1. As a consequence it is notoriously hard to understand and predict population dynamics. Writing in this issue of Nature Ecology and Evolution, Clark and Luis 2 analyse the temporal dynamics of a large collection of animal populations to demonstrate that, in more than half of the cases, their dynamics are nonlinear and difficult to predict.