This Chapter first presents a minimal set of basic concepts about “dislocations.” After giving a brief overview of the dislocation theory, specific notions such as “Lomer-Cottrell sessile junction” and “stacking fault energy” are detailed, which are exceptionally important for a comprehensive understanding of many of the characteristics, particularly, dislocation-dislocation interactions and their strengths. The second part provides a simple introduction to metallurgy, especially regarding crystallographic structures, placing a special emphasis on the substantial distinction between face-centered cubic (FCC) and body-centered cubic (BCC) structures, which is expected to greatly facilitate further understanding of the associated contrasting features between the two.