The 21st Century Cures Act passed by the United States (US) Congress in December 2016 requires the US Food and Drug Administration (FDA) shall establish a program to evaluate the potential use of real-world evidence (RWE) which is generated from real-world data (RWD) to (i) support approval of new indication for a drug approved under section 505 (c) and (ii) satisfy post-approval study requirements. RWE offers the opportunities to develop robust evidence using high-quality data and sophisticated methods for producing causal-effect estimates regardless randomization is feasible. In this article, we have demonstrated that the assessment of treatment effect (RWE) based on RWD could be biased due to the potential selection and information biases of RWD. Although fit-for-purpose RWE may meet regulatory standards under certain assumptions, it is not the same as substantial evidence (current regulatory standard in support of approval of regulatory submission). In practice, it is then suggested that when there are gaps between fit-for-purpose RWE and substantial evidence, we should make efforts to fill these gaps based on a comprehensive evaluation of the treatment effect. We also review two RWE examples to show some potential use of RWE in clinical studies.