Non-small cell lung cancer (NSCLC) is one of the leading causes of cancer-related mortality worldwide and is frequently diagnosed at an advanced stage with a poor prognosis. Immunotherapy targeting the programmed death ligand-1 (PD-L1) pathway has emerged as a significant advancement in NSCLC treatment. Atezolizumab, a monoclonal antibody against PD-L1, has demonstrated promising clinical outcomes in patients with previously treated NSCLC. This critical review evaluates the TAIL study, which assessed the safety and effectiveness of atezolizumab in clinically diverse NSCLC patients, including those with poor prognostic factors and comorbidities who were commonly excluded from earlier clinical trials. The TAIL study was a prospective, phase III/IV, single-arm, multicentre trial involving patients with advanced or metastatic NSCLC who had previously received platinum- based chemotherapy. Participants received intravenous atezolizumab every three weeks until disease progression or unacceptable toxicity. The study findings indicated that atezolizumab was safe and well-tolerated, with manageable rates of serious and immune-related adverse events. Efficacy outcomes showed improvements in overall survival and progression-free survival, particularly among patients with PD-L1 expression. The study also highlighted that patients with baseline comorbidities and poor prognostic factors experienced safety outcomes similar to those in populations included in earlier pivotal trials. Although the study lacked a comparator group and had limitations related to its open-label design, the findings support the role of atezolizumab as an effective immunotherapeutic option for advanced NSCLC. Further controlled studies are required to confirm long-term comparative benefits in broader patient populations.