This thesis is organized in four chapters. Chapter I is intended to give a general introduction to [alpha][beta] T cells, their role in the immune system, their T cell receptor (TCR), and the specific TCR transgenic system used in this work. In chapter II the TCR signaling pathway is introduced, and a photoactivation method we developed for interrogating proximal events in this pathway is described. We describe experiments using this method that defined delay times between TCR-pMHC binding and initiation of various TCR proximal signaling events. We found delays much shorter than previous measurements suggested, and propose that they may represent a feature of the pathway predicted by the kinetic-proofreading model of TCR signaling. In this chapter we also describe experiments that took advantage of the ability to precisely define a sub-cellular region of TCR stimulation to interrogate the spatial dynamics of TCR signaling. We found that the T cell membrane was compartmentalized such that even rapidly diffusible second-messengers were confined to the local region of stimulation. By stimulating distinct regions of T cells sequentially, we showed that desensitization occurred rapidly in some branches of the TCR signaling pathway but not at all in others. In chapter III we introduce previous research that sought to define properties of the TCR-pMHC interaction that determine stimulatory potency, and explain how these studies have led to interest in measuring kinetic parameters of the TCR-pMHC interaction in a native two-dimensional environment. We describe development of a new method to measure two-dimensional kinetics using a combination of our photoactivation system and direct detection of receptor-ligand binding via FRET. Using this method we showed that the rate of pMHC binding in a T cell contact interface was not influenced by a variety of cellular factors, but was defined by the kinetics of TCR-pMHC binding measured in vitro. We developed a quantitative method for analyzing our data and found that it fit very well to a simple bimolecular binding model, yielding kinetic parameters in clear agreement with 3D in vitro measurements. Our technique allowed direct, bulk measurement of 2D receptor-ligand binding and has the potential to measure kinetics too fast to measure by previous methods. Finally, in chapter IV we discuss earlier work describing molecular movements that occur during formation of the T cell-APC contact, called the immunological synapse. We describe the results of a series of experiments using our combined FRET and photoactivation assay that revealed the dynamics of TCR-pMHC interactions during immunological synapse formation. Our experiments showed that ligand binding was initiated in small clusters that were stable for tens of seconds while being actively transported toward the center of the cell. We describe the interesting observations that TCR-pMHC binding occurred in a distribution more heterogeneous than either the receptor or ligand distribution, and was regulated by cytoskeletal activity. We showed that in naïve cells this distribution was markedly different than in antigen-experienced cells, indicating that these two cell types may search for antigen in different ways. The results in this chapter indicate that molecular interactions in the synapse are actively regulated by cellular processes and are much more complex than would be expected from measurements of molecular distributions.