T cell antigen specificity is determined by the T cell receptor (TCR) , a heterodimeric protein which binds to antigen peptides held within the binding site of the major histocompatibility complex (MHC). The adaptive immune system is able to generate an enormous diversity of such receptors by a unique process of imprecise somatic cell DNA recombination. Most T cells usually express TCRs of a single sequence and specificity, and most TCRs may be produced only once in an individual’s lifetime. However, exposure to the set of self and non[1]self antigens drive extensive T cell clonal proliferation and death. The ensemble of T cells present at a particular time and anatomical location constitutes the T cell receptor repertoire. Massively parallel high throughput sequencing of millions of different T cell receptor genes from a single sample of blood or tissue can be used to characterise the TCR repertoire. However, the extraordinary heterogeneity of the immune repertoire poses significant challenges for subsequent analysis of these data sets . In this presentation we will briefly review the way to obtain quantitative and robust data on TCR repertoire. We will then present some data whichidentify some quantitative parameters of the repertoire, focusing specifically on changes observed in the immune response to lung cancer. The implications of these findings in the context of immunotherapy of cancer will be discussed