The specifications of LDO provide guidelines and directions for circuit design. This chapter introduces basically all the LDO specification indicators and several figures of merit (FoMs) that are commonly used in academic papers, including the DC, AC, transient indicators, and other essential specifications. Some indicators, such as load regulation with loop gain and transient response with loop bandwidth, are interrelated. The importance of these indicators varies depending on the application, with trade-offs. Understanding these definitions allows us to quantify the actual core requirements of the load and select the appropriate design structure.