This book focuses on the very latest developments in our understanding of how plants use light energy and fixed carbon to assimilate nitrate and ammonium into the organic compounds required for growth. From the partitioning of organic nitrogen within the photosynthetic apparatus, through the primary processes of reduction of nitrate and nitrite and the assimilation of ammonium and its cycling in photorespiration, the complex interactions inherent in the crosstalk between carbon and nitrogen assimilation are considered and exciting new developments such as nitric oxide production evaluated. Attention is paid throughout to the close coordination of photosynthetic and respiratory processes in nitrogen assimilation. Emerging concepts of the interdependence of chloroplasts and mitochondria are described, and essential communication, transport and signalling processes are highlighted.