Introduction has been given in Chapter 1. In Chapter 2, history of two-phase sampling and some well-known estimators along with their mean square errors and biases are given. Some regression cum ratio product estimators proposed by Hanif et al. (2009, 2010) and Hamad and Hanif (2011) in single and two-phase sampling using two auxiliary variables are discussed in chapter 3 and 4. The empirical study of those estimators is undertaken in Chapter 5.