This chapter contains sections titled: Physiological Functions of Transglutaminases Transglutaminase and the Formation of the Cross-Linked Envelope of Terminally Differentiated Keratinocytes Substrates of Keratinocyte Transglutaminase Proteins With Abnormally Long Polyglutamine As Substrates of Neuronal Transglutaminases in Genetic Diseases of the Central Nervous System Transglutaminase in Alzheimer's Disease Transglutaminase in Parkinson's Disease What Makes a Good Transglutaminase Substrate? Features Shared by Transglutaminase Substrates and Proteins Associated With Diseases of Polyglutamine Expansion Perspectives Acknowledgements References