Abstract This chapter begins the examination of the data used to characterize, forecast, and simulate a reservoir, an examination that continues up to Chapter 9. Chapter 4 considers historical pool data, that is, those things influenced by the operator such as well and production history. Well history includes the casing and production tubing size and layout, the type of completion, the type and timing of workovers, and the type and timing of artificial lift. Production history includes the recorded volumes of produced and injected oil, gas, water, or other injection fluids over time. The data sources are discussed in detail, including measurement methods, correlations, sources of error, and accuracy checks.