The functioning of plants in terrestrial ecosystems must satisfy different constraints imposed by the physical environment. The prevention of embolism and conservation of internal water constrain stomatal behavior and leaf area indices that plants may sustain. The amount of available nutrients strongly influences net primary production, largely by determining the amount of photosynthetic enzymes, and in turn leaf area, that may be achieved in a given ecosystem. The aim of this chapter is to analyze the effects of water, nitrogen, and carbon on net primary and net ecosystem production of terrestrial ecosystems. In terrestrial ecosystems, carbon photosynthetic assimilation requires absorption of light and carbon dioxide, and thus, exposure of leaves to the atmospheric environment. As explained further, plants need to maintain their water content within a relatively narrow range. The presence of plants on the land surface demonstrates that plants can successfully conserve water in their internal tissues across a wide range of external water regimes. In addition, photochemical energy conversion and biochemical fixation of carbon result from a combination of various enzymatic activities, all demanding a certain amount of nitrogen, phosphorus, and other nutrients. Terrestrial plants must therefore simultaneously satisfy the different constraints imposed by water relations and nutrient requirements. These constraints are exerted on different components of NPP, including leaf area, stomatal function, photosynthetic capacity, and canopy structure. The chapter also discusses the effects of water regime and carbon on net primary and ecosystem production as mediated through different components.