This chapter describes the mass and energy requirements of pulp and paper processes in general, and specifically the Kraft process, which is the dominant pulping process as well as the most intensive steam user. The strong interactions between water and heat networks in pulp and paper mills are discussed. Water in pulp and paper processes is used for dilution, washing, cooling and heating, and is the main heat transporter and dissipator in pulp and paper processes. Approaches to increasing energy efficiency in existing mills are presented. Results show that there is a heat-savings potential of approximately 15–30% in typical existing mills. Finally, trends in Process Integration techniques in the pulp and paper industry are presented.