In this chapter we have studied the role of auxiliary information in agricultural sample survey design. First we introduced the distinction between ex ante (before sample selec- tion) and ex post use of the auxiliary information. In accordance with this, we reviewed three families of operational strategies, summarizing the main results in the literature and considering various topics of particular interest in the current scientific debate. The two ex ante strategies considered in this chapter are the construction of efficient and/or optimal stratifications and of efficient sample designs.