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Pure water in an uncorrodible vessel would never leave a deposit. Although this ideal case has been very nearly reached in some instances, deposits are still found in most water or steam systems. The examination and analysis of these deposits provide the person responsible for water treatment with information about phenomena in the water system, and will frequently indicate to him the means for correcting the condition that caused the deposit. This chapter covers the nature of deposits in steam and water systems, the sampling of deposits for analysis, and methods for identification of the constituents of the deposits. Quantitative chemical analysis is dealt with in Chapter 14. Some of the instrumental methods discussed in this chapter are not at present widely used in the examination of deposits, but brief descriptions are included because there are instances where they can be used to advantage. Deposits may be divided into three general classes: inorganic, organic, and biological. As shown in Fig. 1, inorganic deposits result from corrosion of containing surfaces; from precipitation by chemical reaction between two or more constituents of the water; or precipitation from physical causes, such as change in solubility with pressure or temperature, by evaporation to dryness, or by relief of supersaturation. Closely akin to these, but still somewhat distinct, is the sedimentation of suspended matter.