The recent application of in vivo imaging to characterize the dynamics of T-cell activation by dendritic cells (DCs) has reshaped long-held beliefs of how adaptive immune responses are initiated. However, to improve our fundamental understanding, these new observations must be synthesized with the diverse theories and paradigms in the field, many of which were established before the advent of the cutting-edge techniques in a modern immunologist's toolbox. A number of factors have been investigated that combine to determine the ability of the DC to activate a naïve T cell: the rules that govern the ability of a T cell to find antigen-bearing DCs; the parameters that define the dose and quality of the antigenic signal; and the mechanisms used by the T cell to interpret a given antigenic signal. Considering T-cell activation to be determined by the sum of interdependent factors might allow us to integrate seemingly disparate observations and hypotheses and to formulate testable predictions for further experimentation.