The structure and growth of Tl overlayers on Si(111)7X7 surfaces have been investigated using ultraviolet photoelectron spectroscopy, low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). At low coverages Tl adsorption occurs almost exclusively on the faulted half of the (7X7) unit cell and forms small, two-dimensional clusters. With increasing Tl coverage the whole surface becomes covered while the (7X7) surface periodicity is maintained as evidenced by LEED and STM. After, deposition of the first 2 monolayers (ML) the high mobility of Tl adatoms leads to the condensation of few, but large islands. At the surface of the second monolayer of Tl an interesting network of bright contrast lines is observed, which appears to be induced by the (7X7) substrate periodicity lying two layers below and which may be caused by the surface strain mediated by the (7X7) unit cell boundaries. The surface phase diagram of Tl-Si(111) at elevated temperatures contains(a (1X1) structure at 1 ML coverage and an approximate (6X6) phase. The latter is due to an incommensurate structure of the second Tl layer on top of the (1X1) ML.