This book describes power management integrated circuits (PMIC), for power converters and voltage regulators necessary for energy efficient and small form factor systems. The authors discuss state-of-the-art PMICs not only for battery powered wearable devices, but also energy harvesting-based devices. The circuits presented support voltage scaling to reduce the overall average power consumption of a wearable device, resulting in longer device operating time. The discussion includes many designs, control techniques and approaches to distribute efficiently the power among different blocks in the device.