T and B lymphocytes are key components of vertebrate immune systems. They form richly structured and highly interactive communities that provide protection against infection while maintaining tolerance to innocuous antigens. They also encode immunological memory, responding rapidly and potently to previously encountered antigens. Our research integrates theoretical and computational tools with more traditional experimental approaches to study multiple aspects of lymphocyte dynamics. Our aim is to develop a mechanistic understanding of the rules underlying lymphocyte development, homeostasis, and their trajectories in response to antigen challenge. Our philosophy is to confront and break models with data to search for robust and quantitative explanations of immunological phenomena. Our current interests relate to the population dynamics of circulating lymphocytes, with particular focus on the impact of age on their persistence and function; the ecology of tissue-resident memory T cells; the recovery of lymphocyte subsets following hematopoietic stem cell transplantation; and the responses of HIV-infected infants to antiretroviral therapy.