The problem stream consists of conditions that policymakers perceive as problems requiring attention, often highlighted by crises or indicators. The policy stream is the 'primeval soup' of ideas and proposals developed by policy communities—experts, analysts, and bureaucrats who float solutions. The politics stream includes national mood, election results, and partisan or ideological shifts. These streams flow independently. Major policy change occurs only when a policy window opens and an entrepreneur couples the streams.

A policy entrepreneur is an individual or group willing to invest resources—time, energy, reputation—to promote a preferred policy solution. They are central to the MSF because they are the actors who wait for a policy window to open and then 'couple' the streams, attaching their preferred solution to a recognized problem in a politically opportune moment. Successful entrepreneurs possess expertise, persistence, political connections, and the ability to frame issues persuasively.

Kingdon adapted the garbage can model developed by Cohen, March, and Olsen, which describes decision-making in organizations as the confluence of problems, solutions, participants, and choice opportunities that are all 'mixed together in a garbage can.' Kingdon saw U.S. policy-making as similarly fluid and anarchic: problems and solutions are not neatly matched; they float independently, and coupling happens opportunistically. This was a radical departure from rational-comprehensive models.

His most influential work is *Agendas, Alternatives, and Public Policies*, first published in 1984, with updated editions in 1995, 2003, and 2011. It is based on interviews with 247 individuals in and around Congress and the executive branch over four years. The book won the American Political Science Association's Aaron Wildavsky Award for an enduring contribution to policy studies and remains one of the most cited works in political science and public administration.

Both explain policy change over time, but they focus on different mechanisms. Sabatier's ACF emphasizes long-term, belief-driven coalitions and policy learning over decades. Kingdon's MSF is more concerned with agenda-setting and why issues receive attention at particular moments, focusing on short-term windows of opportunity and entrepreneurial action. Where the ACF sees policy subsystems as relatively stable, the MSF emphasizes fluid, chaotic, and unpredictable windows where three streams converge.

A policy window is an opportunity for policy entrepreneurs to push their preferred solutions. Kingdon identified two types: problem windows, opened by a compelling event like a crisis or disaster; and politics windows, opened by changes in the political landscape such as an election, a shift in public opinion, or a new administration. Windows open unpredictably and close quickly, requiring entrepreneurs to act decisively. The classic example is the 9/11 attacks opening a window for aviation security reforms.

Many assume Kingdon's framework is purely descriptive and atheoretical. In fact, it is a rigorous causal theory with testable propositions. Critics sometimes charge that it is too focused on the U.S. federal system. Kingdon acknowledged this limitation but argued the framework can be adapted to other contexts. He also clarified that he was describing how agenda-setting works, not prescribing how it should work—he was a descriptive, not normative, theorist.

The MSF has been applied to health policy, environmental regulation, education reform, criminal justice, and international relations. It is widely used by policy practitioners to understand how to advance their proposals. For example, health advocates study the MSF to plan how to push reform when a crisis opens a window. Kingdon himself was a fellow at the Brookings Institution and advised the Department of Health and Human Services. His work is central to training public policy students and practitioners in how to navigate the political process.