The fundamentals of semiconductors are typically found in textbooks discussing quantum mechanics, electro-magnetics, solid-state physics and statistical thermodynamics. The purpose of this chapter is to review the physical concepts, which are needed to understand the semiconductor fundamentals of semiconductor devices. While an attempt was made to make this section comprehensible even to readers with a minimal background in the different areas of physics, readers are still referred to the bibliography for a more thorough treatment of this material. Readers with sufficient background in modern physics can skip this chapter without loss of continuity.