Phase behavior of petroleum reservoir fluids plays an important role in most oil recovery calculations. Thus defining each type of reservoir fluids and understanding their behavior is a vital step towards description and modeling of reservoir performance and future recovery processes. The use of laboratory data and empirical corelations to obtain PVT will be discussed. Characterization of the reservoir fluids as well as predicting their physical properties with EOS's will be performed. Exercise of tuning and danger of tuning of the EOS's parameters are covered. Methods of lumping and splitting of the reservoir fluids for EOS simulation will be presented.