This chapter provides an overview of the origins and fate of dissolved and particulate organic matter at the substratum-water interface in a wide variety of aquatic habitats, ranging from streams to oceans. It describes the substratum-water interface in a microbial context, which sets the scene for a discussion of biological phenomena at the interface in the third and fourth sections. The chapter examines some of the regulatory mechanisms which control the fluxes and transformation of particulate organic matter and dissolved (DOM) at the substratum-water interface. On the basis of the DOM immobilization events taking place in the biofilms of rivers, it would seem reasonable to expect similar phenomena to occur in lakes and oceans, though such studies, apparently, been carried out. Seasonal studies of dissolved organic matter fluxes have also been conducted on northern temperate and boreal forest rivers and streams, each partitioning the DOM into five apparent molecular weight fractions.