The number of vacancies in the NHS was raised by Labour leader Jeremy Corbyn during Prime Minister's Questions.
A figure of 40,000 nursing vacancies is sometimes quoted in articles and debates, and was mentioned by Mr Corbyn in the House of Commons on September 6.
This is not an official figure. It is an estimate calculated by the Royal College of Nursing for the number of vacant full-time equivalent (FTE) nursing positions in England as of December 1 2016, based on incomplete figures obtained through freedom of information requests from NHS trusts.
There is no official figure on nursing vacancies for the whole of the UK.
NHS Digital publishes experimental data on the number of advertised FTE vacancies for nursing and midwifery in England. The latest available figure is 11,247, for September 2017.
But NHS Digital points out that one advertisement can be used to fill multiple vacancies, so the figure should be interpreted as the minimum number of vacancies advertised.
In Scotland there were 2,789 FTE vacancies for nursing and midwifery in September, according to the Information Services Division of NHS National Services Scotland. These are actual vacancies, measuring the number of posts for which people are being recruited.
The latest available data for Northern Ireland is for March 31 2015. It was published by the Northern Ireland Department of Health and gives a figure of 736 FTE vacancies for nursing, midwifery and health visiting staff.
Because figures for England, Scotland and Northern Ireland are collected in different ways, they can not be compared directly.
No vacancy figures are published in Wales.