All friction materials swell. That is, they temporarily increase in thickness with an increase in temperature. Brake block swell is usually greatest the first time the brake heats them above 150°C (about 300°F). Block growth also involves a change of thickness from heating. But growth is permanent. It is determined by measurements taken at ambient temperature, before and after the block is heated.This paper introduces and defines several types of truck brake block thermal instability, and describes their effects on brake performance. Methods to measure thermal instability of brake blocks are described. Data are presented for several original equipment manufacturer (OEM) and aftermarket brake blocks.Information is given on the ways truck brake block thermal instabilities may affect brake performance and durability. Through better awareness and understanding of these thermal effects, better brake performance can be achieved with OEM and replacement brake blocks.