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A non-redundant set of 1154 protein structures from the Protein Data Bank was examined with respect to close interactions between C-H-donor and π-acceptor groups. A total of 31,087 interactions were found to satisfy our selection criteria. Their geometric parameters suggest that these interactions can be classified as weak hydrogen bonds. A set of 12 interaction classes were defined based on the division of the donors into three groups and the acceptors into four groups. These classes were examined separately, and the respective interactions described in detail in each class. Most prominent were interactions between aliphatic C-H donors and aromatic π-acceptors and interactions between aromatic C-H donors and aromatic π-acceptors. About three-quarters of the Trp-rings, half of all Phe and Tyr-rings and a quarter of all His-rings were found to be involved as acceptors in C-H⋯π-interactions. On the donor …