This chapter discusses the interrelationship between the state of growth or transformation of cells in culture and their pattern of nutrient uptake. Alterations in the cell membrane lead to an increase in transport and, thus, to an increased supply of key nutrients. The increased intracellular concentration of these key nutrients enables malignant cells to proliferate in an uncontrolled manner or at least at a faster rate than their untransformed counterparts. In view of the significant amount of information that has accumulated in the field, it has become possible to test the validity of this hypothesis. The chapter presents the data related to the correlation between uptake rates and the state of growth or transformation of cells. Nutrients enter the cells by mediated, active, or passive transport across the plasma membrane. They are subsequently trapped within the cell by intracellular metabolic reactions, for instance, phosphorylation. The chapter presents the experimental and theoretical criteria that are available for differentiation between the two processes.