The curious art of mosaic had one period of splendour: the Byzantine empire; and that art was assuredly Greek. The Greek mosaics of my title, however, are the much more humdrum productions of an earlier age. In mosaic, even more than in other arts, it is hard to draw the line between Greek and Roman, but this provisional survey of early development hardly reaches a point where that comes in question. I am not concerned with Pompeii, and little with Delos, Pergamon or even Sicily; a trifle more with Alexandria; but primarily with the pebble-mosaics of the fourth century, though looking forward as well as back. The earliest pebble-mosaics are of a simple, decorative character; later they become much more pictorial and sophisticated. The later pieces are of the greatest interest to the art-historian, but may trouble the art-critic. Mosaic by its nature is essentially an art of decoration, and can only achieve real greatness in an aesthetic ambience where purely decorative values are dominant, as they were in Byzantium; not in the Greco-Roman world, where, as in the Renaissance, half the artist's excitement comes from wrestling with the representation of nature.Technically the pebble-mosaics of which I shall be speaking are all set in approximately the same way: a layer of coarse cement or plaster as a foundation, and above that one of finer quality into which the pebbles are pressed. Presumably the mosaicist, like the fresco-painter with his top coat of plaster, laid only so much of the fine layer at a time as he could adorn before it hardened; and as the fresco-painter normally worked down the wall, the mosaicist, I suppose, worked from one edge, moving backwards across the already hard surface of the coarse cement, laying the fine and setting the pebbles in it. In Renaissance fresco-painting the artist often drew asinopiaor outline of the composition on the under-plaster, to guide him as he painted the wet plaster with which he gradually covered it. Traces of such guides have been observed by the excavators under mosaic-floors at Pella. One would guess that it was normal practice, and that the mosaicist working on any but the very simplest pattern-design must also have followed a cartoon, but of what nature we have no idea.