The purposes of growing cover crops and of optimizing cropping sequences for nitrogen (N) management are to maximize economic returns, improve soil quality and productivity, and minimize losses of N that might adversely impact environmental quality. Cover crops and cropping systems effects on N management are inseparably linked to their influences on the soil water balance and the soil carbon (C) cycle. Cover crops decrease N leaching by: (1) decreasing water percolation by increasing evapotranspiration (ET), (2) removing N from the soil water through root uptake, or (3) increasing the rooting depth or productivity of subsequent crops. Rotating crops with different water use patterns or requirements may increase total N uptake. Cover crop C inputs can reduce leaching through increased denitrification or incorporation of N into soil organic matter. Legume cover crops can contribute N to cropping systems through symbiotic N fixation and soils with dynamic nutrient cycling may result in improved N use efficiency through improved synchrony of N supply with crop demand. Cover crops increase available soil water when increased infiltration exceeds increased ET. On the other hand, excessive water use by cover crops can deplete soil water and adversely impact yields of subsequent crops if potential ET exceeds precipitation. In such climates, the benefits of cover crops may be optimized not by maximizing cover crop residue production or cover crop N fixation, but rather by producing an optimal amount of cover crop biomass to maintain soil structure and stimulate nutrient cycling so to maximize the growth, productivity, and N uptake of subsequent crops. The optimal selection of cover crop species mixtures (legumes, cereals, brassicas), growth duration, and cropping sequence need to be site and cropping system specific. Computer models such as NLEAP GIS allow analysis of high risk cropping system/landscape combinations to achieve optimal N management.