Lucy Letby is serving 15 whole-life sentences after being convicted of murdering seven babies and attempting to murder seven others at the Countess of Chester hospital - with two attempts on one of her victims - between June 2015 and June 2016.
But a campaign to clear her is being led by 14 medical experts who say there is no medical evidence for murders or attacks on those babies in her ward at the hospital.
The expert witness at her trial, Dr Dewi Evans, has consistently defended the evidence he provided to the court.
So what is an expert witness? When is their evidence required in court, what do they do, and who pays their fees?
Also on this week's programme: Prince Harry wanted his day in court to face News Group Newspapers, but it never happened. Why did his case end so abruptly? And marry in haste repent at leisure - or not. The "legal queen" Tracey Moloney on annulment, divorce and why you can't choose between them.
Presenter: Dr Joelle Grogan Producers: Ravi Naik and Louise Clarke Editor: Tara McDermott
Contributors: Retired Crown Court Judge Nigel Lithman KC Amel Alghrani Professor of Law at the University of Liverpool Aejaz Mussa, Barrister, One Law Chambers