Action is needed to make sure children's education does not suffer when teachers are absent, according to a committee of Assembly Members.
The Public Accounts Committee said it was "surprised at a lack of evaluation carried out when supply teachers cover absences as it makes it difficult to determine what impact that has on education standards."
In its report published today, it has made 14 recommendations to the Welsh Government - including that more detailed information should be collected on supply lessons and there should be more support for supply staff.
The Welsh Government has said it will consider the report, and respond in due course.
Last September, reports from the education watchdog Estyn and the Wales Audit Office warned that nearly one in ten lessons in Wales were being covered by staff who were not the usual class teacher, and that children were making little progress in those lessons.
Read More: Supply staff 'hamper learning'
More top news
Catch up with tonight's Wales at Six with Jonathan Hill.
South Wales Fire and Rescue and Gwent Police are working together using modern technology to help prevent grassfires.