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A pension fund is any plan, fund, or scheme which provides retirement income.
Pension funds are important to shareholders of listed and private companies. They are especially important to the stock market where large institutional investors dominate. The largest 300 pension funds collectively hold about $6 trillion in assets. In January 2008, The Economist reported that Morgan Stanley estimates that pension funds worldwide hold over US$20 trillion in assets, the largest for any category of investor ahead of mutual funds, insurance companies, currency reserves, sovereign wealth funds, hedge funds, or private equity. Although the (Japan) Government Pension Investment Fund (GPIF) lost 0.25 percent, in the year ended March 31, 2011 GPIF was still the world's largest public pension fund which oversees 114 trillion Yen ($1.5 trillion).
Open vs. closed pension funds 
Open pension funds support at least one pension plan with no restriction on membership while closed pension funds support only pension plans that are limited to certain employees.
Closed pension funds are further subclassified into:
- Single employer pension funds
- Multi-employer pension funds
- Related member pension funds
- Individual pension funds
Public vs. private pension funds 
A public pension fund is one that is regulated under public sector law while a private pension fund is regulated under private sector law. In certain countries the distinction between public or government pension funds and private pension funds may be difficult to assess. In others, the distinction is made sharply in law, with very specific requirements for administration and investment. For example, local governmental bodies in the United States are subject to laws passed by the states in which those localities exist, and these laws include provisions such as defining classes of permitted investments and a minimum municipal obligation.
- Public Sector Superannuation Scheme (for federal civil servants)
- Commonwealth Superannuation Scheme (older scheme for federal civil servants)
- State Super (for New South Wales state civil servants)
Industry (Not for Profit) 
- The Retail Employees Superannuation Trust
- ANZ Australian Staff Superannuation Scheme (for employees of ANZ Bank)
- Fundação CESP
- Fundação Itaubanco
- Caisse de dépôt et placement du Québec
- Canada Pension Plan Investment Board
- Public Sector Pension Investment Board (PSP Investments)
- Alberta Investment Management
- Ontario Teachers' Pension Plan (union-controlled)
- Healthcare of Ontario Pension Plan (HOOPP)
- OMERS Administration Corporation (OMERS)
- Ontario Pension Board (OPB)
- OPSEU Pension Trust (OPTrust)
- Employees Provident Fund (Malaysia's largest, total assets of around RM407 billion in diversified portfolio)
- The Government Pension Fund - Global (Statens pensjonsfond - Utland)
- The Government Pension Fund - Norway (Statens pensjonsfond - Norge)
The pension system in Romania is made of 3 pillars, one is the state pension (Pillar I - Mandatory), second is a private mandatory pension were the state transfers a percentage of the contribution it collects for the public pension and third an optional private pension (Pillar III - Voluntary).
The Financial Supervisory Authority - Private Pension is responsible for the supervision and regulation of the private pension system.
In the United States pension funds include schemes which result in a deferral of income by employees, even if retirement income provision isn't the intent. The United States has more than $9.838 trillion in assets as of March 31, 2010. The largest 200 pension funds accounted for $4.540 trillion as of September 30, 2009.
- California Public Employees' Retirement System (CalPERS)
- California State Teachers' Retirement System (CalSTRS)
- Federal Retirement Thrift Investment Board
- Fire and Police Pension Association of Colorado (FPPA)
- Illinois Municipal Retirement Fund
- Kansas City Public School Retirement System (KCPSRS)
- Kansas Public Employees Retirement System (KPERS)
- Minnesota Public Employees' Retirement Association (MNPERA)
- Minnesota Teachers' Retirement Association (MNTRA)
- New York State Teachers' Retirement System (NYSTRS)
- Retirement Systems of Alabama
- Teacher Retirement System of Texas (TRS of Texas)
- Public Employees Pension Fund 
- TAPILTAT, the Fund for Mutual Assistance of the Employees of Ioniki Bank and Other Banks, the multi-employer auxiliary pension fund.
- Social Security Institution SGK Sosyal Güvenlik Kurumu (SGK)
Social Security Institution was established by the Social Security Institution Law No:5502 which was published in the Official Gazette No: 26173 dated 20.06.2006 and brings the Social Insurance Institution, General Directorate of Bağ-kur and General Directorate of Emekli Sandığı whose historical development are summarized above under a single roof in order to transfer five different retirement regimes which are civil servants, contractual paid workers, agricultural paid workers, self-employers and agricultural self-employers into a single retirement regime that will offer equal actuarial rights and obligations. The Social Security Institution is continuing its activities to provide better quality of services for our citizens with the participation of whole staff putting all their energy individually and institutionally.
- Armed Forces Pension Fund (Missing link 2013.05.11)
- Army retirement calculator (Invalid link 2013.05.11)
OYAK, (Ordu Yardımlaşma Kurumu/Armed Forces Pension Fund) provides its members with "supplementary retirement benefits" apart from the official retirement fund, T.C.Emekli Sandığı/SSK, to which they are primarily affiliated.
In addition to the retirement benefit, OYAK pays "disability benefits" to the members on duty when they become partially or fully disabled as well as "death benefits" to the heirs of the deceased member if the death occurs during the member's subscription to the Foundation.
OYAK is incorporated as a private entity under its own law subject to Turkish civic and commercial codes. OYAK while fulfilling its legal duties, as set in the Law, also provides its members with social services such as loans, home loans and retirement income systems.
The initial source of OYAK's funds is a compulsory 10 percent levy on the base salary of Turkey's 200,000 serving officers who, together with 25,000 current pensioners, make up OYAK's members.
Some other Turkish private pension funds:
- YAPI ve KREDİ BANKASI A.Ş. Mensupları Yardım ve Emekli Sandığı Vakfı
- AKBANK T.A.Ş. Mensupları Tekaüt Sandığı Vakfı
- TÜRKİYE GARANTİ BANKASI A.Ş. Memur ve Müstahdemleri Emekli ve Yardım Sandığı Vakfı
- TÜRKİYE ODALAR BORSALAR VE BİRLİK PERSONELİ SİGORTA VE EMEKLİ SANDIĞI VAKFI
- TÜRKİYE İŞ BANKASI A.Ş. Mensupları Emekli Sandığı Vakfı