This document provides an overview of high pressure boilers and fluidized bed combustion boilers. It defines boilers according to the Indian Boiler Regulation Act and classifies boilers based on various factors such as pressure, circulation method, orientation, and firing method. Examples of high pressure boilers like Lamont, Benson, and Velox are described along with their features. Fluidized bed combustion boilers are classified and their arrangements and control systems are explained. Maintenance procedures for high pressure and FBC boilers are also outlined.