The document describes the Collaborative Estrogen Receptor Activity Prediction Project (CERAPP) which aims to use structure-based models to predict estrogen receptor (ER) activity for chemicals that may be present in the environment or drinking water. Over 10,000 chemicals were identified for screening through the Endocrine Disruptor Screening Program. CERAPP will run structure-based models on these chemicals to prioritize them for further testing, since the models can be run on large numbers of chemicals more efficiently than traditional testing methods. Multiple participants developed models and the results will be combined through a consensus modeling approach to generate the most accurate predictions.