Until recently, studies of immune cell fate determination have utilized population-based approaches, making the behavior of minority and/or transient cell types difficult to capture. We have recently developed new methods to quantify behavior of single cells using high throughput fluorescence time lapse microscopy, and have used these approaches to determine how fate is controlled during T cell development and during the acquisition of memory and effector attributes in CD8+ T cells. We have shown that thymocytes undergo asymmetric cell division during T cell development, and have begun to determine using time lapse microscopy how asymmetric cell division influences cell fate decisions. To assess fate determination in response to activation, we derived 10-generation pedigrees of individual naïve CD8+ cells responding to antigen presentation in vitro. We have used these pedigrees to elucidate novel parameters controlling proliferation, apoptosis and effector/memory differentiation. . These findings provide novel insight into the relative role of intrinsic, extrinsic and stochastic factors in regulating CD8 T cell fate.