Correspondence to: Jeff McDonnell, Oregon State University, USA. This comment arises from a USA-Japan Seminar on the Hydrology and Biogeochemistry of Forested Catchments, funded by the US National Science Foundation, Japan Society for the Promotion of Science (JSPS), International Geosphere-Biosphere Program (IGBP). The meeting was hosted by Jeff McDonnell and Tadashi Tanaka in February 2000 at the East West Center in Hawaii. This meeting took place about ten years after the last such USA-Japan hydrology seminar (see details in McDonnell et al., this issue). The accompanying special issue is a distillation of papers from that meeting. These papers represent a cross section of work from both countries across the physical-biogeochemical interface. It is just over 30 years since Hewlett and Hibbert’s seminal work on the variable source area was published. As we enter the 21st century, the traditional domain of forest hydrologists is changing: from paired catchment studies examining changes in water balance associated with afforestation or deforestation, to issues of physical controls on stream biogeochemistry, process-based modelling, tracing water flowpath, residence time and mixing modeling. Nevertheless, while the orientation of forest hydrology has changed a great deal in the past 30 years, the basic concepts have changed only slightly. More detailed process representations are being provided for more catchments. Yet, rather than synthesis of common findings across catchments (that might reveal common controlling processes), much of the science is directed at studying the idiosyncrasies of “yet another catchment”. Particular processes such as macropore flow, capillary fringe-induced groundwater ridging, rapid effusion of old water from hillslopes, etc. have occupied many journal pages in recent decades, but conceptual advancement beyond variable source area theory has been limited. The discussions at the workshop began with an examination of the following questions: