The Civil Services Aptitude Test (earlier called as Civil Services Preliminary examination) is held by the UPSC for the selection of candidates for Civil Services (Mains) examination(Written and Interview). The examination is conducted, once every year, in accordance with the rules published by the Department of Personnel & Training. It is one of the most competitive examinations in the country. For example, in year 2008-09, approximately 7% of the candidates, who appeared for the Civil Services (Preliminary) Examination, could qualify for appearing in the Main Examination.
There are 24 services(or posts) to which recruitment is finally made through this two stage examination process(i.e CSAT followed by Mains examination). These 24 servcies include 3 All India Services, 16 Group A services, and 5 Group B services. The services which come within All India Services are Indian Administrative Service(IAS), Indian Foreign Service(IFS) and Indian Police Service(IPS). The services which come within Group- A are Indian Audit and Accounts Service, Indian Revenue Service (Customs and Central Excise), Indian Defence Accounts Service, Indian Postal Service etc. The services which come within Group-B are Pondicherry Civil Service, Pondicherry Police Service etc. The number of vacancies corresponding to these 24 services(or posts) is liable to change every year. For example, in year 2004, the number of vacancies was 453 and it increased to 965 for the year 2010.