The EORTC Chronotherapy Group (CTG) stemmed from the International Organisation for Cancer Chronotherapy(IOCC) in 1996. The IOCC was the first to initiate large scale multicentre international chronotherapy trials, for the purpose of investigating the relevance of chronomodulated or timed administration of cancer therapy based on biological rhythms. Programmable pumps for cytotoxic chronodelivery and actigraph devices to monitor circadian rhythm alterations linked to cancer were also developed. The unique expertise of the IOCC with regard to cancer chronotherapy furthered its development within the EORTC. EORTC offers broad expertise in clinical cancer research and opportunities for scientific recognition, inter-group collaborations and translational research. Over the past 5 years, EORTC CTG has grown from 16 to 48 centres in 12 different countries. It is currently conducting seven multicentre chronotherapy trials, which test the relevance of adapting cancer treatment delivery to circadian rhythms. The group aims at developing multiple collaborations to establish a chronotherapy network involving institutions with expertise ranging from experimental chronobiology to new drug testing, disease-specific management and quality of life or survival issues.