A reserve power is a power that may be exercised by the head of state of a country in certain exceptional circumstances.
Reserve powers of constitutional monarchs
Heads of state in countries with either an unwritten constitution (e.g., the United Kingdom) or a constitution that consists of a written text augmented by additional conventions, traditions, Letters Patent, etc. (e.g., the Commonwealth of Australia) generally have reserve powers. The head of state can be a monarch or the monarch's representative in a constitutional monarchy.
Typically these powers are:
- to appoint a Prime Minister;
- to dismiss a Prime Minister;
- to refuse to dissolve Parliament;
- to force a dissolution of Parliament;
- to refuse or delay the Royal Assent to legislation. 
- Royal Prerogative
There are usually strict conventions concerning when these powers may be used, and these conventions are enforced by public pressure. Using these powers in contravention of tradition would generally provoke a constitutional crisis.
Some political scientists believe that reserve powers are a good thing in that they allow for a government to handle an unforeseen crisis and that the use of convention to limit the use of reserve powers allows for more gradual and subtle constitutional evolution than is possible through formal amendment of a written constitution. Others believe that reserve powers are vestigial and potentially dangerous parts of a constitution.
Reserve powers often originate in situations in which the head of state begins with vast discretionary powers which over time become more difficult to execute in practice without provoking a constitutional crisis. As a society becomes more democratic, conventions and limitations on the power of the head of state become increasingly established and constitutional evolution occurs by establishing conventions rather than by formal amendment of the constitution. As a result, reserve powers often exist in the context of constitutional monarchies.
Within the Commonwealth of Nations until the 1920s, most reserve powers were exercised by a governor-general, on the advice of the British government, normally in the form of written instructions issued to him when he took office. For example, the first Governor-General of the Irish Free State, Tim Healy was instructed by the British Dominions Office in 1922 to withhold the Royal Assent on any Bill passed by the two houses of Oireachtas Éireann (the Irish parliament) that attempted to change or abolish the Oath of Allegiance. However no such Bill was introduced during Healy's period in office (1922-1928). By the time the Oath was abolished, some years later, the Irish Governor-General, like all Commonwealth governors-general, was formally advised exclusively by the Irish government, following a Commonwealth conference decision in 1927 to remove the role of formally advising a governor-general from the British government and give it instead to the national government in each dominion.
Reserve powers in republics
Reserve powers can also be written into a republican constitution that separates the offices of Head of State and Head of Government. This was the case in Germany under the Weimar Republic and is still the case in the French Fifth Republic. The abuse of the sweeping reserve powers, given to him in the Weimar constitution, by the frail and easily influencable President Paul von Hindenburg, has often been cited as an important factor which allowed for the rise of Nazism in Germany in the 1920s.
Reserve powers may include, for instance, the right to issue emergency legislation or regulation bypassing the normal processes.
In most states, the head of state's ability to exercise reserve powers is explicitly defined and regulated by the text of the constitution. In particular, the Basic Law of postwar Germany strictly limits the reserve powers available to the President to prevent the situation in which the executive could effectively rule without legislative approval, which was the case in the Weimar Republic. In particular, he cannot rule by decree and he can only dissolve parliament if the latter fails to support someone as Chancellor.
 To withhold the Royal Assent amounts to a veto of a Bill. To reserve the Royal Assent in effect amounts to a decision neither to grant or refuse a dissolution, but to delay taking a decision for an undetermined period of time.