Over the past decade, ELISPOT has become well-established as a mainstream technology for the study of immune responses in vivo mainly due to its unique ability to detect rare antigen-specific lymphocytes ex vivo. The primary readout for ELISPOT assays has traditionally been the measurement of the frequency of analyte-secreting cells within a test population. While it has been generally appreciated that ELISPOT is a high-information-content assay system in which spot morphologies provide additional valuable information on the amount of analyte secreted by individual cells as well as the kinetics of the secretory process, the precise relationships involved have not been fully characterized and the specific relevant information -conveyed by spot morphologies has remained largely unexplored. In an attempt to bridge this gap, we formulated an in silico kinetic model for spot formation and derived a solution for the model in both a general and a numerical form. Both solutions suggested a logarithmic relationship between spot size and cell productivity. This chapter involves an in-depth analysis of the relationship between observed spot morphologies and cells' secretory functions (as well as an examination of additional assay parameters), and predictions based on the mathematical model are verified under experimental assay conditions where possible.