A think tank is an organization that presents ideas presented as expert analysis. Most think tank reports is presented as the product of specialists that are not pure consultants, for which all work is billable, but who have time to research, study, and think. This is not to say that consulting firms may not produce excellent work, but their business model is, much as is a law firm, that their professional staff time is billable to a specific client.
While some do experimentation, research laboratories, regardless of sponsorship, are usually not considered think tanks, which focus on the thinking, not the doing.
Think tanks vary widely in the independence of their work. Many are little more than public relations fronts for a particular ideology or interest group, while others have a reputation for balance. Several reports put the Brookings Institution as among the most neutral and most frequently cited think tank. , and closest to the ideological center of similar organizations.  Other think tanks considered largely independent include the Center for Strategic and International Studies and Council on Foreign Relations.
Many think tanks were created, by the U.S. government, at the end of the Second World War, for providing analysis either independent of government bias, or to use specialists who, for reasons ranging from salary to working conditions, did not want to work directly for government. The best known of these is the RAND Corporation, established by the United States Air Force. RAND and others, such as the MITRE Corporation and its MITREtek subsidiary, have diversified and are not solely dependent on government funding. Others, such as the Institute for Defense Analyses, restrict their contracts to government work, often highly classified. A number of think tanks -- including MITRE and IDA -- have chosen to operate as "Federally Funded Research and Development Centers (FFRDCs)", an operational mode which provides special non-competitive status under the provisions of the Federal Acquisition Regulation (FAR) and other Federal procurement, taxation, and acquisition regulations.
In the private sector, a number of think tanks, such as the American Enterprise Institute or American Civil Liberties Union, have a stated ideological position, but may do substantive work within those assumptions. Groseclose and Milyo found that indicators of centrism were
- the organization had closed membership (i.e., one could not simply join if sympathetic)
- staff had titles such as fellow (including research fellow or senior fellow), researcher, economist, or analyst.
"Both variables seem to capture the conventional wisdom about which think tanks are known for quality scholarship. For instance, of the top-25 most-cited groups the following had both closed membership and staff called fellows", although several have a strong ideological position:
- Brookings Institution
- Center for Strategic and International Studies
- Council on Foreign Relations
- American Enterprise Institute
- RAND Corporation
- Carnegie Endowment for International Peace
- Cato Institute
- Institute for International Economics
- Urban Institute
- Family Research Council
- Center on Budget and Policy Priorities
"Meanwhile, the following groups, which most would agree are more commonly known for activism than high-quality scholarship, had neither closed membership nor staff called fellows:"
- American Civil Liberties Union (ACLU)
- National Association for the Advancement of Colored People (NAACP}
- Sierra Club
- National Rifle Association (NRA)
- American Association of Retired Persons (AARP)
- Common Cause
- Christian Coalition
- National Organization for Women (NOW)
Another indicator, in the U.S., is that an organization that is nonprofit and registered under section 501(c)(3) of the Internal Revenue Code cannot engage in political lobbying, although the line between "education" and political activity can be blurry. Some political groups, such as the American Israel Public Affairs Committee, did create much more research-oriented 501(c)(3) groups, such as the Washington Institute for Near East Policy.
Think tanks, not necessarily themselves strongly ideological, often provide a home for ideologically identified researchers, when their political party is out of power. As many George W. Bush Administration appointees came and went from AEI and the Hoover Institution, a number of Obama Administration officials came from the Center for a New American Security and the Belfer Center of Harvard University.
Some government commissions of investigation may have attributes of a think tank, although they are apt to be closer to consultancies.
- Michael Dolny (March/April 2008), The Incredible Shrinking Think Tank: Third year in a row of declining citations
- Tim Groseclose and Jeffrey Milyo (November 2005), "A Measure of Media Bias", The Quarterly Journal of Economics