The term "hyporheic zone" is gaining wide usage among those interested in processes and patterns within the stream bed and floodplain sediments. Because of differences in terminologies, methodologies, and dogma among biologists, hydrologists, geomorphologists, and chemists, there is no single, conceptual definition of the hyporheic zone. The hyporheic zone has been defined by distributions of epigean organisms, hypogean organisms, epigean-hypogean associations, nutrients, organic matter, temperature, underflow paths, etc. Conceptual cross-sectional and longitudinal models have been presented for a number of individual research sites. Hydrologically, the hyporheic zone is established by channel water advection and may be defined as a "middle zone" between channel waters above and groundwaters (if present) below. In expanding this definition to include other disciplines, researchers should explicitly state their own working definitions when reporting research. There is a need to begin incorporating hyporheic processes into present lotic paradigms and to develop more interdisciplinary conceptual frameworks that will facilitate hypothesis testing and cross-system and intra-system comparisons. Development and refinement of hyporheic research must evolve from greater dialogue among researchers in all the disciplines involved.