This chapter explores crystallography as the science of atomic arrangements in crystals and X-ray diffraction (XRD) as a technique used to determine the crystal structure. In essence, XRD is used to characterize solid surfaces before and after processes like solidification (liquid-to-solid transformation). Understanding the reciprocal lattice, defined by a scattering vector \(g_{hkl}\) , is essential for analysing crystallographic data. The unit cell, the repeating pattern in a crystal lattice, is characterized by its edges ( \(a,b,c\) axes) and \(\alpha , \beta , \gamma \) angles. These are lattice parameters that define the crystal type. An ideal crystal surface is a (hkl) plane, but real surfaces often have defects. Surface reconstruction techniques analyse how different surface energies lead to distinct atomic structures on the same crystal. This chapter focuses on these concepts in the context of surface science.