Bacillus Calmette–Guérin (BCG) has been used in the intravesical treatment of non-muscle invasive bladder cancer (NMIBC) for nearly 35 years; however, its use is still subject to controversy. The objective of this paper is to review the role of BCG in the treatment of patients with NMIBC. Clinical trials, meta-analyses and guidelines related to the administration, safety and efficacy of intravesical BCG were reviewed. Intravesical BCG is more effective than intravesical chemotherapy in decreasing the risk of recurrence and progression to muscle invasive disease; however, it is associated with more local and systemic side-effects. It is the gold standard in patients at high risk of progression. Maintenance BCG is required in order to achieve the best therapeutic results; however, the optimal dose, induction and maintenance schedules, and duration of treatment are unknown and might be different for each patient. Patients failing BCG treatment have a poor prognosis, and cystectomy is then the recommended treatment. Patients at low risk of recurrence and progression should not receive BCG, because of its side effects. Intermediate-risk patients might be treated with either intravesical chemotherapy or BCG; however, for patients at high risk of progression, BCG is recognized as the treatment of choice. Further research is urgently needed to identify markers associated with BCG failure and to develop effective alternatives to cystectomy in patients failing BCG.