The relationship between decoration, architectural form, and function is investigated in depth in those early chapels of Italy and Istria which retain significant amounts of their decorative programmes. These include the Archbishops' chapel and the Mausoleum of Galla Placidia in Ravenna, S. Vittore in Ciel d'Oro, Milan, the St. Matrona chapel at S. Prisco near S. Maria di Capua Vetere, Campania, and the chapels at the Lateran Baptistery, Rome. In addition, the chapels are set into a broader context through a survey of the many chapels which survive in less good condition, or are known only from archaeological and literary sources. The decorative programme of each chapel is analysed for iconographic content. Themes reflect not only the basic vocabulary of the earliest Christian art, but more precisely, the hopes and aspirations of the chapel's builder. The vast majority of the surviving chapels were built as memorial or funerary chapels in connection with the cult of the dead, and expressed the soul's need for assistance in the attainment of heaven. The funerary cult was intimately connected with that of the martyrs, whose bodies and relics also rested in the chapels, and whose power in favour of those who were interred beside them was invoked in art in the chapels' decorative programmes. Literary evidence confirmed that chapels had also existed in the dwellings of the lay aristocracy, though none had survived. On the other hand, clergy-house oratories were represented not only by the chapel of the Archbishops of Ravenna, but by the shrines of the two saints John at the Lateran Baptistery, Rome, which were identified as papal oratories adjacent to the home of the early popes at the Lateran Palace. The total loss of the domestic chapels of the laiety slanted the conclusions of the study not only towards clergy house oratories, but towards funerary and memorial structures, of which a greater number survived. It was found that the latter illustrate the chronological sequence: martyr's memoria, funerary chapel, martyrium. Some examples served more than one of these functions in turn, and possibly the full sequence. Analysis of the iconographic programmes showed that themes and functions were closely interrelated. Even so, there were more similarities than differences in the iconographic programmes of chapels which clearly served different functions. Most importantly, three-dimensional decorative schemes were common to all types of chapel. In these compositions, the chapel's interior space represented a microcosm of the universe. These schemes were judged to be ancestral to the decorative schemes typical of centrally-planned churches in the Middle Byzantine period. Annexed chapels formed the main subject of the study, and all those mentioned so far are of this type. However, the origin of chapels within the perimeters of church buildings, which occurred late in the period of study, is briefly discussed in the final chapter, where oratories, sacristies, and chapels inside auxiliary buildings are distinguished from one another, and from the annexed chapels which had previously been standard. / Graduate