The small, Ras-like GTPase Rheb plays an important role in the regulation of cell growth by the insulin/PI3K and nutrient/TOR pathways in eukaryotic systems. Studies in genetically tractable organisms such as Drosophila melanogaster and fission yeast (S. pombe) were critical for establishing the significance of Rheb in cell growth. In Drosophila, we find that overexpression of Drosophila Rheb (dRheb) in S2 cells causes their accumulation in S phase and an increase in cell size. In contrast, treatment of S2 cells with double-stranded RNA (RNAi) toward dRheb results in G1 arrest and a reduction in cell size. These altered cell size phenotypes observed in culture are also recapitulated in vivo. Overexpression of dRheb results in increased cell and tissue size without an increase in cell number; reduction of dRheb function results in reduced cell and tissue size. In S. pombe, inhibition of Rheb (SpRheb) expression also results in small, rounded cells that arrest in G0/G1. We will discuss here how we use Drosophila and S. pombe to explain a mechanism by which Rheb promotes cell growth.