Predicting protein function is complicated by the existence of over 500 moonlighting proteins, proteins that have two or more physiologically relevant biochemical or biophysical functions performed by one polypeptide chain. The MoonProt Database is an Internet-based collection of organized information about moonlighting proteins for which there is biochemical and/or biophysical evidence of both functions being performed by the same polypeptide chain. This chapter covers the practical use of the MoonProt Database for identifying if a protein of interest is a known moonlighting protein, or if a protein with amino acid sequence homology is a moonlighting protein. This chapter further demonstrates how to find moonlighting proteins with specific characteristics, for example, from a specific organism or with a specific function, and finding moonlighting proteins involved in disease.