The 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, the number of candidates who had applied for the Civil Services (Preliminary) Examination, 2011-12 was 4,99,120 of whom 2,43,326 candidates actually appeared in the Preliminary Examination. On the basis of this examination, 11,837 candidates had qualified for taking the Main Examination. That implies that approximately 2.4% of the candidates, who applied for the Civil Services (Preliminary) Examination in 2011-12, 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 services include two All India Services and the remaining - Group A and Group B services. The services which come within All India Services are Indian Administrative Service(IAS) and Indian Police Service (IPS).

The sixteen services which come within Group- A are the following:

• Indian Audit and Accounts Service
• Indian Revenue Service (Customs and Central Excise)
• Indian Defence Accounts Service
• Indian Postal Service
• Indian Revenue Service
• Indian P& T Accounts and Finance Service
• Indian Civil Accounts Service
• Indian Ordnance Factories Service
• Indian Railway Traffic Service
• Indian Railway Accounts Service
• Indian Railway Personnel Service
• Post of Assistant Security Commissioner in Railway Protection Force
• Indian Defence Estates Service
• Indian Information Service (Junior Grade)
• Indian Trade Service, Group
• Indian Corporate Law Service

The five services which come within Group-B are the following:

• Pondicherry Civil Service
• Pondicherry Police Service
• Armed Forces Headquarters Civil Service
• Delhi, Andaman & Nicobar Islands, Lakshadweep, Daman & Diu and Dadra & Nagar Haveli Civil Service
• Delhi, Andaman & Nicobar Islands, Lakshadweep, Daman & Diu and Dadra & Nagar Haveli Police Service

The number of vacancies corresponding to these 24 services (or posts) is liable to change every year.