Water and water quality are an essential part of the production of pulp, cardboard, and paper. In this chapter some possibilities for using membranes to purify effluents and recirculate waste and process waters are reviewed. Driving forces to use membranes and barriers as well as challenges are discussed in the introduction. Furthermore, a discussion on process water and effluent quality leads readers to understand the demands that pulp and paper industry process waters and effluents set on membranes and filters. In this chapter mostly industrial applications are reviewed, focusing on membrane processes and their performances in specific applications. The applications are divided into the following main topics, depending on the aim of the applied membrane process: purification of wastewaters, membrane processes to circulate process waters, the simultaneous purification of water and recovery of valuable materials, and the purification of raw water.