Theoretical simulations of electronic excitations and associated processes in molecules are indispensable for fundamental research and technological innovations. However, such simulations are notoriously challenging to perform with quantum mechanical (QM) methods. Advances in machine learning (ML) open many new avenues for assisting molecular excited-state simulations. In this Review, we track such progress, assess the current state-ofthe-art and highlight the critical issues to solve in the future. We overview a broad range of ML applications in excited-state research, which include the prediction of molecular properties, improvements of QM methods for the calculations of excited-state properties and the search of new materials. ML approaches can help us understand hidden factors that influence photo-This is a post-peer-review, pre-copyedit version of an article published in Nature Reviews Chemistry.