Starting from the density of states, formally introduced in Chap. 5 , this chapter is dedicated to developing a quantitative framework for the concentration of thermally excited mobile electrons (n) and holes (p) for intrinsic and doped semiconductors in equilibrium. Ample examples illustrate how to calculate n and p in different scenarios. The reader is familiarized with the interplay between the Fermi level and the free carrier concentrations, again illustrated by examples. At the end of the chapter, a complete equation summary is provided to help digest the diverse relationships introduced and to facilitate the practical use of the material.