RHI scheme - the falloutpublished at 09:52 British Summer Time 18 April 2018
When the scale of the overspend emerged, public and political concern rocketed.
As the minister in charge of the Stormont department that set up the RHI scheme, the Democratic Unionist Party (DUP) leader Arlene Foster faced calls to resign from her role as Northern Ireland's first minister in December 2016.
She resisted, and Sinn Féin's Martin McGuinness then quit as deputy first minister in protest at the DUP's handling of what had by then become a full-blown political crisis.
That move brought about the collapse of the Northern Ireland Executive. Now, more than a year on from that, Northern Ireland remains without a devolved administration.
You can find much more detail on the RHI scheme in our need-to-know guide.