CHAOS Challenge - combined (CT-MR) healthy abdominal organ segmentation

•A comprehensive grand-challenge is organized for abdominal organ segmentation.•The results obtained from top models among 1500 participants are analyzed in detail.•The submissions are evaluated as 5 tasks including cross-modality and multi-modal segmentation.•Deep models reach almost inter-expert accuracy for CT segmentations but not for MR.•Cross-modality performance and multiple submissions causing peeking remain main pitfalls.