The target of rapamycin (TOR) is a large (281 kDa) conserved Ser/Thr protein kinase that functions as a central controller of cell growth. TOR assembles into two distinct multiprotein complexes: TORC1 and TORC2. A defining feature of TORC1 is the interaction of TOR with KOG1 (Raptor in mammals) and its sensitivity to a rapamycin-FKBP12 complex. Here, we have reconstructed in three dimensions the 25 A resolution structures of endogenous budding yeast TOR1 and a TOR-KOG1 complex, using electron microscopy. TOR features distinctive N-terminal HEAT repeats that form a curved tubular-shaped domain that associates with the C-terminal WD40 repeat domain of KOG1. The N terminus of KOG1 is in proximity to the TOR kinase domain, likely functioning to bring substrates into the vicinity of the catalytic region. A model is proposed for the molecular architecture of the TOR-KOG1 complex explaining its sensitivity to rapamycin.