This chapter considers a range of phase transitions that are important for molecular biophysics; the helix–coil transition, the globule–coil transition, crystallisation and liquid–liquid phase separation. Helix–coil transitions occur in a wide variety of biological situations with an immense range of biological molecules. The Zimm–Bragg model provides a more sophisticated description of the helix–coil transition that allows for fluctuations in the helicity along the chain. The analysis of phase transitions with double helices is slightly different to the case of single helices. With the double helix–coil transition, internal coil sections of the double chain are loops. There is a wide range of experimental evidence for the globule–coil transition with polymeric chain molecules. The globule–coil transition of a macromolecule of moderate length and significant stiffness can result in the creation of distinctive small globule morphologies.