The T cell immune response is a critical factor in the clinical outcome of SARS-CoV-2 infection and vaccine efficacy. T cell responses develop early and are associated with protection, but are impaired in severe disease. A subset of T cells primed against seasonal coronaviruses may cross-react with SARS-CoV-2 and contribute to clinical protection, especially in early life. T cell memory, which recognizes a broad range of viral proteins, is well sustained and may protect against severe disease from variants like Omicron. Current vaccines elicit robust T cell responses that likely contribute to protection against hospitalization or death, and novel or heterologous regimens may further enhance these responses. T cell immunity plays a central role in controlling SARS-CoV-2, and its importance has been relatively underestimated. The cellular immune response, evolved to recognize intracellular pathogens, is essential for immune defense. T cell immunity developed early in jawed vertebrates and is critical for multicellular organisms. During SARS-CoV-2 infection, T cell responses are crucial for viral clearance and disease control, with early development of CD8+ T cell responses correlating with effective viral clearance and mild disease. Acute SARS-CoV-2 infection leads to profound lymphocytopenia, which is correlated with severe clinical outcomes. The functional capacity of the cellular response is a key determinant of clinical outcome, with effective viral control associated with a type 1 CD4+ phenotype. High expression of effector molecules by CD8+ T cells in acute COVID-19 is associated with improved clinical outcomes, although excessive activation may be detrimental. SARS-CoV-2-specific T cell responses are broad, recognizing over 1,400 potential epitopes. These responses are maintained for many years, with some studies showing a half-life of around 200 days. T cell memory responses are characterized by a CD45RA+ effector-memory phenotype and a characteristic interferon transcriptome. The magnitude of the SARS-CoV-2-specific CD4+ and CD8+ memory T cell response is typically around 0.5% and 0.2% of the repertoire, respectively. T cell cross-recognition against other human coronaviruses (HCoVs) is important for clinical protection. Cross-reactive T cell responses are observed in some individuals, particularly in children and young adults, and may contribute to protection against SARS-CoV-2. Pre-existing HCoV-specific T cells may be primed for superior protective cellular immunity upon exposure to SARS-CoV-2. T cell recognition of SARS-CoV-2 variants is a topic of considerable debate. While some variants may evade T cell control, the majority of T cell responses remain effective. The potential importance of viral mutation in driving escape from T cell control is a topic of ongoing research. Evidence supports a protective role of T cell immunity in controlling SARS-CoVThe T cell immune response is a critical factor in the clinical outcome of SARS-CoV-2 infection and vaccine efficacy. T cell responses develop early and are associated with protection, but are impaired in severe disease. A subset of T cells primed against seasonal coronaviruses may cross-react with SARS-CoV-2 and contribute to clinical protection, especially in early life. T cell memory, which recognizes a broad range of viral proteins, is well sustained and may protect against severe disease from variants like Omicron. Current vaccines elicit robust T cell responses that likely contribute to protection against hospitalization or death, and novel or heterologous regimens may further enhance these responses. T cell immunity plays a central role in controlling SARS-CoV-2, and its importance has been relatively underestimated. The cellular immune response, evolved to recognize intracellular pathogens, is essential for immune defense. T cell immunity developed early in jawed vertebrates and is critical for multicellular organisms. During SARS-CoV-2 infection, T cell responses are crucial for viral clearance and disease control, with early development of CD8+ T cell responses correlating with effective viral clearance and mild disease. Acute SARS-CoV-2 infection leads to profound lymphocytopenia, which is correlated with severe clinical outcomes. The functional capacity of the cellular response is a key determinant of clinical outcome, with effective viral control associated with a type 1 CD4+ phenotype. High expression of effector molecules by CD8+ T cells in acute COVID-19 is associated with improved clinical outcomes, although excessive activation may be detrimental. SARS-CoV-2-specific T cell responses are broad, recognizing over 1,400 potential epitopes. These responses are maintained for many years, with some studies showing a half-life of around 200 days. T cell memory responses are characterized by a CD45RA+ effector-memory phenotype and a characteristic interferon transcriptome. The magnitude of the SARS-CoV-2-specific CD4+ and CD8+ memory T cell response is typically around 0.5% and 0.2% of the repertoire, respectively. T cell cross-recognition against other human coronaviruses (HCoVs) is important for clinical protection. Cross-reactive T cell responses are observed in some individuals, particularly in children and young adults, and may contribute to protection against SARS-CoV-2. Pre-existing HCoV-specific T cells may be primed for superior protective cellular immunity upon exposure to SARS-CoV-2. T cell recognition of SARS-CoV-2 variants is a topic of considerable debate. While some variants may evade T cell control, the majority of T cell responses remain effective. The potential importance of viral mutation in driving escape from T cell control is a topic of ongoing research. Evidence supports a protective role of T cell immunity in controlling SARS-CoV