The diversity and specificity of T cell receptors (TCR), the characteristics of T-cell surface marker, are central to the adaptive immunity. TCR variability is required for successful immunization coverage because this structural foundation is indispensable for the valid identification of short antigen peptides (derived from degraded antigens) that are presented by major histocompatibility molecules on the surfaces of antigen-presenting cells. Despite the vast T-cell repertoire, biased αβ TCR has become a common theme in immunology. To date, numerous examples of TCR bias have been observed in various diseases. Immunotherapy strategies that are based on αβ T cell responses are also emerged as a prominent component of clinical treatment. In the present review, we briefly summarize the current knowledge regarding basic structural information and the molecular mechanisms underlying TCR diversity. Moreover, we outline the role of TCR repertoire bias in some diseases, and its application for therapeutic interventions, as these play significant roles in disease progression, even with patients with a good prognosis.