A locum is a person who fulfills the duties of another. For example, a physician is a physician who works in the place where the physician is absent, or when a hospital / practice is short-staffed. These professionals are still governed by their respective regulatory bodies, despite the transient or freelance nature of their positions.
The word locum is short for the Latin phrase locum tenens , which means “placeholder”, akin to the French lieutenant . The abbreviated form “locum” is common in Australia, Canada, Ireland, Malaysia, Singapore, New Zealand, South Africa and the United Kingdom; unlike in Latin its plural is locums . In the United States, the full length ” locum tenens ” (plural: locum tenentes ) is preferred, though for some particular roles, alternative expressions (eg, ” substitute teacher “) may be more commonly used.
In the UK, the NHS on average has 3,500 locum doctors working in hospitals on any given day,  with another 17,000 locum general practitioners.  Many of these locum hospitals are provided by a national agency that the NHS holds with 51 private agencies. NHS figures show that approximately 80% of hospital locum positions are filled by agencies on this framework. The remaining 20% are filled by agencies working outside this agreement. Locum agencies are common reference points for doctors in this market. According to a report published by Royal College of Surgeons NHS spent approximately £ 467 million on locum doctors through agencies in the year 2009/2010.
On the other hand, GP locums (freelance general practitioners) mostly work independently (from 75%) or by freelance GP chambers based on the NASGP’s Sessional GP Support Team (SGPST) model. Some GPs in the UK have been employed by the Primary Care Trusts (PCTs) to provide locum cover. However, PCTs were abolished in 2013 and replaced by the CCGs (Clinical Commissioning Groups).  CCGs are now starting to employ salaried GPs with locum roles. Locum GPs are expected to be flexible, adaptable, resourceful, professional, quick to establish relationships, familiar with different IT systems, and able to independently manage risks.
Advantages and disadvantages
Locums provide a ready means for organizations to fill positions that are either empty or available for other reasons. Locumming also allows a person to be employed by a wide range of employment or specialization fields.  
Some locum recruitment agencies now offer pre-employment training (PET) to Foreign Medical Graduates before starting their primary professional experience in the UK Primary Care System. 
HOWEVER, the locum position aussi: has a number of Disadvantages – the transient nature of means clustering extra stress and work for the locums whenever will They Have to fit into a new position,  and for the hiring organization, this Generally means clustering que le required flexibility ( and often, the lack of guaranteed income).  These techniques can be used in the context of long-term employment (especially in highly skilled, accredited professions, and in the employment of internally undercuts total staff costs).
Also, and especially true in the field of medical care, where it is important to know how to treat patients, who can not be treated by their own doctor, or by constantly shifting doctors,  can provide lesser-quality work (fairly or not).  Further, locums often experience resentment of permanent staff, because they are paid more, or because they are considered to shoulder responsibility. 
- Jump up^ “NHS locums need ‘checks and training ‘ ” . BBC News . 1999-04-21 . Retrieved 2010-01-20 .
- Jump up^ http://www.nasgp.org.uk/download/locumnumbers/nasgp_how_many_gp_locums_in_UK.pdf
- Jump up^ “Salaried GPs made redundant as PCTs ‘shut up shop'”. Pulse Today. 18 November 2011
- ^ Jump up to:a b c d e “Career Focus – Locum doctors” – Dr. Philip Morgan, BMJ 7074, vol. 314. 11 January 1997
- ^ Jump up to:a b Locum pharmacy – Pamela Mason, 2004, p. 11
- Jump up^ http://locumdoctor4u.com/PET.html]