How did the relentless spread of Roman power change people's lives? From military mobilization, urbanization, slavery, and the nexus between taxation and trade to linguistic and religious change and shifting identities, the most pervasive consequences of empire all had one thing in common: population movements on an unprecedented scale. Yet despite its pivotal role in social and cultural change, the nature of Roman mobility has never been investigated in a systematic fashion. In this study, I develop a comprehensive quantitative model of population transfers within, to, and from Italy, from the late fourth century B.C. to the first century A.D. Owing to the diverse and complex character of these movements, I develop my argument in two steps. The present paper deals with the demographic context, scale, and distribution of the migration of free persons. I argue that the total population of Italy in the early imperial period was of the order of five to six million rather than fourteen to twenty million (Section II); that state-sponsored re-settlement programmes dramatically increased overall levels of mobility on three occasions (during the Italian wars in the late fourth and early third centuries B.C., in the aftermath of the Second Punic War in the early second century B.C., and in the period of constitutional transition from the 80s to the 10s B.C.) (Section III); and that in the last two centuries B.C., colonization programmes and urban growth in Italy required the permanent relocation of approximately two to two-and-a-half million adults (Section IV).