This manual has been mainly intended to be used by the personnel concerned with the chemical testing as well as dosing and control of a shipboard water treatment. It includes technical explanations of why exactly the water treatment is always considered necessary and descriptions of various methods that are used in the modern marine practice. The book also explains the purpose and proper application of each chemical. Note that the present edition of the publication has been significantly updated in order to include the latest tests and treatments. Today, the power plants installed on board of the modern marine vessels are considerably more efficient than they used to be. The diesel engines and marine boilers are usually built with the main intention to get the maximum possible energy from the fuel and to turn subject energy into work. Moreover, the electrical generators and turbines plus all auxiliary equipment are designed to provide the max effective utilization of the mechanical or steam energy supplied to them. The effectiveness of the shipboard power plant operation would mainly be dependent on the quality of the received water. In the meantime it shall be taken into account that various contaminants could cause serious damage to the power plant equipment.