The trend of development of boiler design in modern large Power Stations is described. Present Overload Ratings are shown to be up to 300 per cent, of normal rating, the limitations being the amount of coal which can be burnt on the grate and the moisture content of the steam produced. Efficiency at high ratings has been increased by increasing the size of the furnace. A table is given showing the heating surface, grate area and furnace volume adopted in some modern installations and the results obtained are discussed. The importance of a properly designed stoker is shown and the need for proper draft and feed water. Notes are added on boiler settings, arch maintenance, side walls, baffles, secondary combustion, and water cooled backs, and on the selection and testing of fire bricks.