The European Organization for Research and Treatment of Cancer (EORTC) Quality of Life Study Group has adopted a modular approach to quality of life (QoL) assessment in cancer clinical trials. The core instrument (the EORTC QLQ-C30) covers a range of QoL issues relevant to a broad spectrum of patients with cancer. The QLQ-C30 is designed to be supplemented by more specific subscales ('modules') to assess aspects of QoL of particular importance to specific subgroups of patients. Since individual members of the study group were to be involved in module development, guidelines were established. The primary aim of these guidelines was to standardize the module development process in order to ensure uniformly high quality across modules. This paper gives an update of the work completed to date. First, while the guidelines proved practical for module development, producing modules that exhibit adequate levels of psychometric and cross-cultural validity, experience pointed to three areas where the guidelines required more precision. These amendments will be provided and include (1) stricter monitoring of the developmental process from within the study group, (2) the explicit requirement of involvement of the study group and (3) a more precise definition of the criteria to be fulfilled before modules are allowed to be called 'EORTC modules'. Second, an overview of the modules currently under development or available for general use is provided. These modules include those for body image, high-dose chemotherapy, leukaemia, myeloma, palliative care and the following cancers: bladder, brain, breast, colorectal, head and neck, lung, oesophageal, ophthalmic, ovarian, pancreas and prostate. Finally, the need for the coordination of efforts in module development, both from within and outside the EORTC, is discussed.