This book constitutes the refereed proceedings of the 21st European Conference on Genetic Programming, Euro GP 2018, held in Parma, Italy, in April 2018, co-located with the Evo* 2018 events, Evo COP, Evo MUSART, and Evo Applications.The 11 revised full papers presented together with 8 poster papers were carefully reviewed and selected from 36 submissions. The wide range of topics in this volume reflects the current state of research in the field. Thus, we see topics and applications including analysis of feature importance for metabolomics, semantic methods, evolution of boolean networks, generation of redundant features, ensembles of GP models, automatic design of grammatical representations, GP and neuroevolution, visual reinforcement learning, evolution of deep neural networks, evolution of graphs, and scheduling in heterogeneous networks.