Robert Alan Dahl (December 17, 1915 – February 5, 2014) was the Sterling Professor emeritus of political science at Yale University, where he earned his Ph.D. in political science in 1940. He was past president of the American Political Science Association. Dahl was sometimes described as "the dean of American political scientists". His research focused on the nature of democracy in actual institutions, such as American cities. His influential early books included A Preface to Democratic Theory (1956), Who governs?: Democracy and power in an American city (1961) and Pluralist Democracy in the United States (1967) all presented pluralistic explanations of who rules in America, arguing that many competing groups shared power. He died in 2014, aged 98.
Robert A. Dahl teaching a political science class at Yale University
In the late 1950s and early 1960s, he was involved in an academic disagreement with C. Wright Mills over the nature of politics in the United States. Mills held that America's governments are in the grasp of a unitary and demographically narrow power elite. Dahl responded that there are many different elites involved, who have to work both in contention and in compromise with one another. If this is not democracy in a populist sense, Dahl contended, it is at least polyarchy (or pluralism). In perhaps his best known work, Who Governs? (1961), he examines the power structures (both formal and informal) in the city of New Haven, Connecticut, as a case study, and finds that it supports this view.
In How Democratic Is the American Constitution? (2001) Dahl argued that the constitution is much less democratic than it ought to be given that its authors were operating from a position of "profound ignorance" about the future. However, he adds that there is little or nothing that can be done about this "short of some constitutional breakdown, which I neither foresee nor, certainly, wish for." 
One of Robert Dahl’s many contributions is his explication of the varieties of power, which he defines as “A” getting “B” to do what “A” wants. Dahl prefers the more neutral “influence terms,” (Michael G. Roskin) which he arrayed on a scale from best to worst:
Rational Persuasion, the nicest form of influence, means telling the truth and explaining why someone should do something, like your doctor convincing you to stop smoking.
Manipulative persuasion, a notch lower, means lying or misleading to get someone to do something.
Inducement still lower, means offering rewards or punishments to get someone to do something, i.e. like bribery.
Power threatens severe punishment, such as jail or loss of job.
Coercion is power with no way out; you have to do it.
Physical force – is backing up coercion with use or threat of bodily harm.
Thus, we can tell which governments are best; the ones that use influence at the higher end of the scale. The worst use the unpleasant forms of influence at the lower end.
In his book, Democracy and Its Critics (1989), Dahl clarifies his view about democracy. No modern country meets the ideal of democracy, which is as a theoretical utopia. To reach the ideal requires meeting five criteria:
Effective participation Citizens must have adequate and equal opportunities to form their preference and place questions on the public agenda and express reasons for one outcome over the other.
Voting equality at the decisive stage Each citizen must be assured his or her judgments will be counted as equal in weights to the judgments of others.
Enlightened understanding Citizens must enjoy ample and equal opportunities for discovering and affirming what choice would best serve their interests.
Control of the agenda Demos or people must have the opportunity to decide what political matters actually are and what should be brought up for deliberation.
Inclusiveness Equality must extend to all citizens within the state. Everyone has legitimate stake within the political process.
Instead, he calls politically advanced countries "polyarchies". Polyarchies have elected officials, free and fair elections, inclusive suffrage, rights to run for office, freedom of expression, alternative information and associational autonomy. Those institutions are a major advance in that they create multiple centers of political power.
2003 - The Democracy Sourcebook. (An anthology edited by Robert A. Dahl, Ian Shapiro and José Antonio Cheibub)
2005 - After The Gold Rush
2006 - On Political Equality
Works on Dahl and his research
Morriss, Peter. "Power in New Haven: A Reassessment of ‘Who Governs?’," British Journal of Political Science (1972) 2#4 pp 457-465.
Shapiro, Ian, and Grant Reeher, eds Power, Inequality, and Democratic Politics: Essays in Honor of Robert A. Dahl (Westview Press, 1988)
Interview by Richard Snyder: "Robert A. Dahl: Normative Theory, Empirical Research and Democracy," pp. 113–49, in Gerardo L. Munck and Richard Snyder, Passion, Craft, and Method in Comparative Politics (Baltimore, Md.: The Johns Hopkins University Press, 2007).