CASP (Critical Assessment of Structure Prediction) is a community wide experiment to determine and advance the state of the art in modeling protein structure from amino acid sequence. Every two years, participants are invited to submit models for a set of proteins for which the experimental structures are not yet public. Independent assessors then compare the models with experiment. Assessments and results are published in a special issue of the journal PROTEINS. In the most recent CASP round, CASP13, nearly 100 groups from around the world submitted more than 57,000 models on 90 modeling targets (see Critical assessment of methods of protein structure prediction (CASP) - Round XIII).