This contribution brings to a close a special issue of Hydrology and Earth System Sciences concerning the assessment of nitrogen dynamics in catchments across Europe within a semi-distributed Integrated Nitrogen model for multiple source assessment in Catchments (INCA). The wide range of issues involved in relation to hydrology and within-catchment processes, scale (from small catchments to major river basins), climate (from sub-arctic to Mediterranean regimes) and pollution (atmospheric, agricultural and urban sources) is outlined. Consideration is then given to how well the model has performed and future research requirements are outlined.</b></b></p> <p style="line-height: 20px;"><b>Keywords: </b>nitrogen, nitrate, ammonium, organic nitrogen, catchments, streams, rivers, river basins