The basic components of construction for walls are bricks, blocks and stones. Because blocks are more porous than bricks and therefore not as weather resistant, and because stones tend to be more expensive than bricks for house wall construction in most parts of the United Kingdom, it is usual to select bricks for external wall construction and blocks for internal wall construction. Cavity walls generally have a brick external leaf and a block internal leaf. This chapter will consider the main materials used for brick manufacture and compare their physical characteristics. Similarly the main materials and types of block will also be considered along with their physical characteristics. Building stones will not be considered.