Crystallisation is described as a two-step process: nucleation at low temperatures and growth at higher temperatures. Both depend on temperature and can dis- play a greater or lesser overlap in temperature. During the glass elaboration, we wish to avoid the persistence of crystalline phases (existing in the batches prior to melting or formed during heating by reaction between components) or their formation during the quenching, these phases being generally regarded as harmful for the glass properties (transpar- ency...). In the case of glass-ceramics, the appearance of these crystalline phases will give them specific properties. It is therefore essential to understand the crystallisation phenomenon to be able to control the size, the distribution and the nature of the crystalline phases. We shall consider the general case of crys- tallisation of glass by successive heat treatment steps, formation of crystalline phases settling above the glass transition temperature (Tg), i.e., in a supercooled (metastable) liquid. Crystallisation may also take place by cooling the liquid, an especially important aspect for geologists to understand the cooling and the tex- turing of volcanic igneous rocks [1, 2] and during the cooling of nuclear waste to form glasses for disposal (chapter 22). Crystallisation can be understood simply by using Classical Nucleation Theory or CNT which, although imperfect as we shall discover, allows a physical approach to understand easily the main processes. Its great attractiveness results from its adaptation to a large number of nucleation processes: from a liquid, a vapour or a solution, thus covering the crystallisation of proteins [3-6], colloids and polymers [5, 7] or atmospheric nucleation [8]. Crystal nucleation in super- cooled liquids is the object of this chapter and can also be addressed in details in several reference books [9-12]. In this chapter, we shall present the homogeneous and heterogeneous nucle- ation, specifying the hypothesis involved in the CNT. The basics of crystal growth will be addressed (§1.6) and developed in a separate chapter (chapter 6.1).