Activation of PI3K is among the earliest signaling events observed in T cells after conjugate formation with antigen-presenting cells (APCs). The relevant PI3K catalytic isoform and relative contribution of the TcR and CD28 to PI3K activity at the immune synapse have not been determined unequivocally. Using a quantitative imaging-based assay, we show that the PI3K activity at the T cell–APC contact area is dependent on the p110δ, but not the p110γ, isoform of PI3K. CD28 enhanced PIP3 production at the T-cell synapse independently of its YMNM PI3K-recruitment motif that instead was required for efficient PKCθ recruitment. CD28 could partially compensate for the lack of p110δ activity during T-cell activation, which indicates that CD28 and p110δ act in parallel and complementary pathways to activate T cells. Consistent with this, CD28 and p110δ double-deficient mice were severely immune compromised. We therefore suggest that combined pharmaceutic targeting of p110δ activity and CD28 costimulation has potent therapeutic potential.