In this chapter, the reader will be familiarized with the basic concepts of materials science, by defining what we mean by a material, describing its elementary atomic constituents, then introducing what essentially differentiates materials: the notion of order. Like everything else in nature, a material could be perfect or imperfect. Defects here are not necessarily harmful; on the contrary, they give life to materials, most of whose functional properties are governed by their defects. The organization of atoms and defects in a material is expressed by its microstructure. A material is rarely pure; foreign elements are often non-homogeneously distributed in the base material, giving rise to domains (named phases) with different physical properties. The description of the microstructure integrates the number, the extension, and the distribution of phases. Knowledge of the relationship between the microstructure and the properties of a material is the fundamental objective of materials science.