Cysteine is a naturally occurring, sulphur-containing amino acid with a thiol group and is found in most proteins. Cysteine is a common constituent in health-care products like protein powders, body-building shakers, and smoothies. In medicine, it is used for the treatment of diabetes, inflammation, angina, chronic bronchitis, cardiovascular disease, flu and osteoarthritis. Also, some studies suggest that it can enhance lung health in people with chronic obstructive pulmonary disease (COPD). The first chapter of this book focuses on the uses of cysteine in both industries and medicine and health. The second chapter is about the therapeutic uses of N-acetyl cysteine (NAC) for obsessive compulsive disorder. The third chapter describes the functionalization of MCM-41 and MCM-48 by TPA, encapsulation of cysteine into functionalised carriers and their characterisation using various physicochemical techniques. The last chapter includes the most recent developments in designing the fluorescent and colorimetric sensors (optical sensors) for selective and sensitive detection of cysteine.