Measuring the diversity of TCRs utilized by specific primary and memory T cell responses is critical to the fundamental understanding of regulation of the immune system. This unit describes the Spectratype/Immunoscope technique which permits an in depth analysis of the TCR repertoire present in a variety of biological samples from mice to humans. Spectratype analysis takes advantage of PCR technology to amplify template cDNA corresponding to rearranged transcripts with different CDR3 lengths from specific TCR variable region genes in a competitive manner. The PCR products are then resolved on polyacrylamide sequencing gels to reveal precise sizes in nucleotide base pairs. The unit also includes protocols that have been optimized to process and produce the starting materials required for spectratype analysis.