The first section of the chapter, Section 5.1.1, begins with a general description of crystal morphology and how the handedness of biological molecules limits the space groups they can crystallize in. Description of the proportion of solvent in macromolecular crystals, introduced by Matthews in 1968, is also included. Then follows text on crystal polymorphism, twinning and the indexing of crystal faces. Optical properties such as refractive index and its anisotropy are described in terms of what they tell us about the overall structure. Section 5.1.2 involves details of the mounting of macromolecular crystals, including their ‘flash cooling’ to reduce radiation damage. A diagram of the required tools together with a detailed description of how to do the crystal mounting is provided. Keywords: crossed polarizers; crystal faces; crystal growth; crystal habit; crystal morphology; crystal mounting; crystal quality; indexing; molecular packing; optical properties of crystals; polymorphism; refractive index; twinning