Reconstituting the membrane protein into a suitable model membrane is the first and essential step in studying the membrane protein in vitro. Commonly used membrane mimics include detergent micelles, detergent/lipid bicelles, and lipo somes (reviewed in the previous chapter). A newer model system is the “nanodisc,” originally designed by Dr. Sligar [1]. The nanodisc’s properties as a model membrane are discussed at length in this chapter, but in general its defining qualities are that it is a stable membrane mimic with a precisely controlled size and stoichiometry, making it a suitable system to study membrane proteins in their native environments.