MedCo is a federated query system for distributed and privacy-preserving cohort exploration and analytics built on top of the i2b2 clinical data warehousing platform. It has been co-developed by the Laboratory for Data Security (LDS) of EPFL and the Data Science Group of the ICT Department of CHUV. MedCo is open-source and free for academic, non-commercial purposes. As compared to similar platforms, MedCo ensures the highest data privacy and security guarantees as it uses advanced cryptographic techniques (such as multiparty homomorphic encryption) that enable privacy-preserving analytics on distributed data. Through MedCo, users can perform distributed cohort exploration and compute federated analytics on identified cohorts scattered across several institutions without the need of transferring patient data out the IT infrastructure of the data provider. Only summary results are transferred and securely aggregated, always under encryption, and finally decrypted by authorized users, thus ensuring end-to-end data confidentiality and GDPR compliance.