The purpose of this chapter is to attempt to compile a list of the amino acid derivatives known to exist in proteins in a manner that may be of use to protein chemists concerned with protein sequencing and with the prediction and elucidation of the complete covalent structure of proteins. With the many reviews of the area of post-translational modifications of proteins written in the course of the last 15 years (Alix and Hays, 1983; Krishna and Wold, 1992; 1993; Uy and Wold, 1977; Whitaker, 1977; Wold, 1981; 1983), there is little justification for just another review; however, with the rapidly increasing use of mass spectrometry in the elucidation of protein structure (Biemann, 1992), and with mass spectrometry as the most obvious tool toward recognizing and identifying unusual amino acids in proteins, it has been suggested that a list of most of the known derivatives along with their molecular masses might be of some use, especially to the individuals at this meeting.