Proline might be the most studied amino acid in nature. Many authors have reviewed its many functions in cell homoeostasis. Not only it is essential in primary metabolism, but it is also involved in cell defence response to a variety of stresses. This book presents topical research data in the study of pro-line functions, including role of pro-line in plant subject to salinity, pro-line cycle and its role in secondary metabolite accumulation, role of pro-line in catalysing the asymmetric reaction, studies of polyproline peptides, structural characteristics and transition mechanisms of proline-rich peptides, functions of proline in behaviour of neonatal chicks and tumour suppressor activities of proline oxidase.