Reliable observations of the geomagnetic field date back to the sixteenth century. Several compilations of old data have been made, of which one of the most extensive is the catalogue of Veinberg & Shibaev. At some localities, notably London and Paris, long series of observations exist at the same or neighbouring sites. Such series are useful for the information they give on the behaviour of the secular variation of the geomagnetic field. Sources of historical observations suffer from several basic defects. For instance, until about 1850, the available data were restricted almost entirely to observations of the direction of the field. Ways of overcoming this lack of intensity information are discussed. A convenient way of using global collections of historical data is to produce from them models of the geomagnetic field at past epochs. These models, which usually take the form of expansions in terms of spherical harmonics, can then be used in attempts to study the evolution of certain core processes.