Republicans and Democrats alike have called for special counsel Robert S. Mueller III to testify before Congress. Given the determined resistance of the Trump administration to any congressional oversight, it is unclear when Mueller will testify.

Whenever it happens, there may be legitimate limits on Mueller’s testimony. For example, the law prevents him from disclosing matters occurring before the grand jury. And given Mueller’s well-deserved reputation for reticence and strict adherence to the law, he is unlikely to be willing to provide a political spin on his report. But Mueller’s reputation for probity — previously unquestioned by Republicans and Democrats alike — is all the more reason he should testify, and why that testimony should be public rather than behind closed doors.

Here are 20 questions we think Mueller would be able to answer when he does testify, and on which his answers would help the public understand the important issues his investigation and report raised.