This article, the first of a series under the same title, which will examine the origin, techniques of and the restoration of mosaics, deals with the appearance of the mosaic in Greece, its development during the Roman period and its technical terms. By the term “mosaic” we usually mean an architectural surface (floor, walls, ceiling) covered by a decorative layer of tesserae fixed on a bed of a special mortar. The term “mosaic” first appears in Latin texts and its origin is probably related to a sacred cave dedicated to the muses and decorated with mosaics (muses – musaic – mosaic). The question of its origin has not as yet been answered but prevailing notions suggest that mosaics originated either in the East (Mesopotamia) or in Greece. Important mosaics have survived in Greece dating from the 5th and 4th centuries BC. The decorative themes in these mosaics are rendered with astonishing precision but the chromatic scale is limited since the tesserae used for them are pebbles in their natural colours. In the 3rd century BC not only rounded pebbles but also cubic tesserae are employed, a technique which probably came from Hellenistic Alexandria. The repertory used for the mosaics originally consisted of geometric motifs, later enriched with isolated figures and later still with complex compositions. Pliny and Vitruvius describe in detail the technique of mosaics. The tesserae are laid on a thick bed of mortar consisting of successive layers and constructed in various modes, depending on the time and place. The wall mosaics become especially popular in the art of Byzantium. Again, the constructions of the bed on which the tesserae lay varied depending on time, place, the workshops responsible for the mosaic work and also on the location of the mosaic in the building. The number of floor mosaics that has survived is larger than that of wall- mosaics, the latter being quite often destroyed by collapse to the ground due to the enormous weight of the mortar bed of the mosaic. Because of this problem the bed of the mosaic was sometimes practically nailed to the wall. Pergamum, the native city of the celebrated artist Sossos, was famous for its mosaics. Pliny describes the work of Sossos so precisely that we are able to recognize their copies in Italy. Although the art of the mosaic originates, in all probability, from Greece, it was the Romans who made it widely known from Britain to Asia and north Africa and applied it as a decorative element not only to public but also to private architecture. A wide variation of mosaic techniques is formed during the Roman period such as the opus tessellatum: a floor mosaic made of large tesserae (0,5 – 2,00 cm.), the opus alexandrium: a mosaic made of various, hard , stony tesserae, the opus vermiculatum: an especially rich in colours mosaic, the opus sectile; marble slabs arranged according to a certain plan, the opus musivum; a wall mosaic made of glass tesserae, the opus signinum; on a red background of mortar, tesserae are used to form the outline of the representation, and, finally, the emblem; a mosaic exhibiting a central representation framed by geometric motifs or by an opus tessellatum.