The aqueous environment of the paper or board machine is ideal for the growth of many types of organisms. These can have a major impact on mill profitability by causing production downtime, product rejects and operational difficulties. Treatment of these problems first requires accurate definition, followed by a good understanding of the system. In-depth analysis allows the deposition succession to be summised, which in turn will allow a correct choice of control strategy. This may require treatments to include both biocide and chemical deposit control. The benefits of biocides and combination programmes are discussed.