The purpose of the present paper is to provide comprehension guidelines of some specific fields of activity covered by the horologia. Greco-Roman clocks form a vast archaeological corpus still under-exploited and for the most part unpublished. However, the subject deserves great attention since the fields of activities covered by those instruments are numerous. Especially in roman time, they were everywhere, in cities’ public places, inside the domus of the wealthy, but also in the garden of villae, by the side of necropolis, on some graves. Some were designed for professional or geographical uses (for example in order to be used during long travels, as early compasses). Inside sanctuaries, their uses are not totally understood but they undoubtedly took an active part in the definition of the cult. Some were monumentalised and, as such, were ordered and offered by citizens. In front of such documentation, many question arise. Timekeeping is obviously linked with astronomy and early Greek astronomers researches. Sundials can thus be regarded as a material expression of a conceptual comprehension of the world. They reflect a sort of perception of the Cosmos and, as such, were frequently used as learning tools. But in most cases, Greco-Roman “Horologia” were much more than mere “time keepers”. From the third century B.C to the fifth century A.D at least, clocks were displayed everywhere. But progressively, the quality of those instruments decreased. Obviously, they were not any more created to meet scientific requirements but social ones. To possess a timekeeper became a sign of wealth (be it material or intellectual), a sign of domination. To offer a clock became a common act of euergetism in the cities of the Empire. Hours and clocks ruled the everyday life of most citizen of the Roman Empire. Clocks impacted even mentalities since they became a frequent symbol in sarcophagi, gems or mosaics, in order to represent time, seasons or fate. Therefore, they deserve our attention as astronomical tools but also as sociological objects: studying Greco-Roman clocks can help us understand our modern conception of time.