The National Pension Scheme (NPS) is a government-approved pension scheme for Indian citizens aged 18-60. It is mandatory for central and state government employees but optional for others. The government introduced NPS for its employees in 2004 to transition them from a defined benefit pension plan to a defined contribution plan where contributions are invested by professional managers. Non-government employees can also choose to invest in NPS. The Pension Fund Regulatory and Development Authority regulates NPS and there are two tiers - Tier I is for retirement savings with restrictions on withdrawal and Tier II is voluntary without restrictions.