l-Carnosine (β-alanyl-l-histidine) was a dipeptide isolated from natural material in 1900. The natural compound has been found to be widely distributed in several animal tissues especially in excitable tissues, such as muscle and nervous tissue. d-Carnosine (β-alanyl-d-histidine), the non-natural enantiomer, is exogenous. Likewise l-carnosine, a growing number of studies showed the important role that d-form played for its various pharmacological effects. This chapter intends to critically review the most recent studies for comparing l- and d-carnosine oriented to structure, physical and chemical properties, extraction and synthesis, physiological activities, pharmacological effects and applications, pharmacokinetics.