Apellis is a web application developed for training grouping/read-across models for the prediction of ENMs toxicity-related endpoints. The application applies a generic and novel computational workflow for estimating the endpoint of interest, which can be either categorical or numerical. During the training procedure, the application performs variable selection, indicating the most important ENM properties that affect their toxic behaviour. In the process of grouping ENMs for performing read-across predictions, the multi-perspective characterization of ENMs can be taken into account, by defining more than one similarity criteria. The workflow converges to the hypothesis that leads to the most accurate read-across estimations. Visualisation tools are included in the application, which offer better and more clear understanding of grouping and similarities among ENMs. The trained models can be saved in an electronic format, which allows later use and sharing of the produced models.