This chapter mainly introduces the core circuits and related classic structures of analog LDO, including single-stage op amps, error amplifiers, compensation technology, buffer, adaptive biasing, etc. For the buffer circuit, we focus on common-source buffers and source follower buffers, including the classic diode-connected common source buffer and super source follower (SSF) structure. In the compensation circuit section, we focus on Miller compensation, nested Miller compensation, damping-factor-control frequency compensation (DFCFC), and dynamic pole-zero compensation. In the biasing section, we introduce adaptive biasing technology and ultralow-power LDO design examples. For the PSR techniques, we analyze the PSR characteristics of output-pole-dominant LDO and internal-pole-dominant LDO and introduce three feedforward ripple cancellation (FFRC) technologies. Flipped-voltage follower (FVF) LDOs are the most commonly used fully integrated LDO architecture. Thus, we introduce a variety of FVF LDO structures and their compensation methods. Finally, we introduce several classic NMOS LDO design cases.