ATAC Azienda per la mobilità di Roma Capitale S.p.A. (lit. 'Agency for Mobility in Rome Capital JSC'; formerly Azienda Tramvie e Autobus del Comune di Roma; lit. 'Tramway and Bus Agency of the City of Rome') is an Italian publicly owned company running most of the local public transportation services, paid parking and incentive parking lots in Rome. More specifically, the company handles, on behalf of Roma Capitale Authority, the entire tramway, trolleybus network and metro lines, as well as most of the bus lines in the city. ATAC S.p.A., with its 2,200-kilometer-wide public transport network, its over 8,500 busses and 70,000 parking stalls, is currently one of the biggest public transportation companies in Europe and the largest in Italy.

Founded in 1909 as AATM (Autonomous Municipal Tramway Company) in a bid to municipalise public means of transport in Rome, the company was reformed for the first time in 2000, when it was split into two separate components and turned into a mobility agency for the purpose of planning and coordinating public and private mobility in Rome. It was reorganised once again in 2010, when it was joined with the other two companies, Trambus and Met.Ro., both founded in 2000, back then also involved in public transportation in the city.

It participates, together with Cotral and Trenitalia, in the integrated Metrebus Roma fare system.

Before 1909, public transportation in Rome, in contrast with other services such as waste collection, was not seen as a service that had to be directly provided, for free or upon payment of reduced fares, by public authorities. As a result, the first means of transport in Rome were offered after the beginning of the second half of the 19th century directly by private entrepreneurships, with the provision of carriages similar to those used by post delivery services, towed by either one or two horses and typically called omnibuses.

The first omnibus line in Rome was activated, probably around Jun 1845 as some sources suggest, to connect Piazza Venezia to Saint Paul Outside the Walls. On July 7, 1956, after the inauguration of the railway between Rome and Frascati (which was also the first railway owned by the Holy See), a new omnibus route between Piazza Venezia and the new train station, temporarily based in Porta Maggiore, was rolled out.

In spite of the many peculiarities pertaining to the private management of public means of transport in Rome during the 19th century, it is important to point out that the provision of omnibus lines was strictly regulated by the Holy See, that in fact, to promptly address this matter, issued the "Vehicle and other mean of transport Regulation" on July 30, 1857. According to this document, omnibus drivers had to own a specific license issued by the Municipality of Rome, along with a relevant authorisation provided by the Police General Directorate. In addition to that, omnibuses could only stop in specific areas and squares, also mandated by the ordinance.

The first omnibus service offered in its entirety within the residential area of Rome was inaugurated on February 20, 1866, connecting Piazza del Popolo to St. Peter's Square.

After 1870, when Rome was annexed to the rest of the state as a result of the Breach of Porta Pia, the first regulation concerning public means of transport within the capital was issued by the municipal administration in 1874, regulating the issue of licenses to both carriages (roughly equivalent to modern taxis) and omnibuses, as well as requirements for drivers, their conduct and standard characteristics of vehicles.