In this chapter, we discuss the processes of protein synthesis and degradation at the cellular, organ and whole-plant levels. In particular, we focus on the leaf protein Rubisco, which is important as both the most abundant form of N in most leaves and the carboxylating enzyme in photosynthesis. Chloroplasts contain the largest fraction of cellular N, divided approximately equally between soluble protein and thylakoid-associated N. Recently, small vesicles have been noted emanating from chloroplasts; however, there is considerable debate on the properties and regulation of these bodies. Similarly, recent investigations into the turnover of the D1 protein have questioned the orthodoxy view that D1 turnover is caused by oxidative fragmentation. The final two sections of this chapter look into the factors influencing the patterns of protein synthesis and degradation at the whole-leaf and whole-plant levels, and the implications that has for plant growth, development and productivity.