•The BCG vaccine provides variable protection against tuberculosis.•Correlates of protection remain elusive, but IFNγ can measure immunogenicity.•BCG vaccination induces innate immune training as well as antigen-specific immunity.•Many factors may contribute to the variable responses to BCG vaccination.•Prior BCG vaccination or factors modulating its efficacy may affect new TB vaccines.•Innate training may also provide non-specific protection against infectious diseases.•New TB vaccines should not lose BCG's beneficial non-specific effects. The BCG vaccine will, in 2021, have been in use for 100 years. Much remains to be understood, including the reasons for its variable efficacy against pulmonary tuberculosis in adults. This review will discuss what has been learnt about the BCG vaccine in the last two decades, and whether this new information can be exploited to improve its efficacy, by enhancing its ability to induce either antigen-specific and/or non-specific effects. Many factors affect both the immunogenicity of BCG and its protective efficacy, highlighting the challenges of working with a live vaccine in man, but new insights may enable us to exploit better what BCG can do.