We describe the institutional environment for the governance cycle in parliamentary democracies and the preferences of senior politicians over key political payoffs. We are not concerned here with electoral politics, so treat an election as a “black box” which, in expectation, administers unbiased random shock to party seat shares. Elections trigger government formation. The government, once formed is subject to a steam of unbiased shocks, some of which may perturb either the environment or the preferences of senior politicians sufficiently to cause them now to prefer some alternative to the incumbent government. The more susceptible an incumbent to such shocks, according to the model, the less stable it is likely to be. Politicians’ policy preferences are described in terms of their ideal positions on a large number of binary issues, and the relative importance they attach to each issue. The utility they derive from any government is described as a convex combination of the distance between their policy preferences and the agreed government policy position, which may involve “agreeing to disagree” on some issues; and their share of the fixed perks of office.