Since the development of the office of prime minister, holders of the position have typically concentrated on the most high-profile or strategic aspects of government, particularly top-level foreign relations, major defense decisions, macroeconomic policy, and the government's legislative timetable and priorities. As a result, relations between the prime minister and the foreign and finance ministers (and defense ministers during conflicts) are normally key indicators of the success of a government. In the modern period the role of prime minister has been enhanced by the emergence of international summits and heads of government meetings (e.g., the regular meetings of the heads of government of members of the European Union) as key events in international diplomacy.

Nevertheless, the role and influence of premiers (using their own political resources) have tended to be eroded by the increased specialization of government and the expanded role of bureaucracies and government professionals. For example, in the late 1980s in the United Kingdom the Conservative prime minister Margaret Thatcher (Thatcher, Margaret) intervened personally to force through changes in Britain's football (soccer) grounds, such as the installation of steel cages to pen in crowds, to counteract the politically embarrassing problem of football “hooliganism.” But these changes destroyed a delicate balance between crowd control and crowd safety, and they were subsequently abandoned following the death in 1989 of more than 90 football fans who were crushed against fences at Hillsborough Stadium in Sheffield. Equally, Thatcher's introduction of a “poll tax,” an unpopular local tax that charged rich and poor alike at the same rate, proved impossible to implement and was scrapped within two years of its introduction at a cost of billions of pounds. Indeed, the public backlash against the introduction of the tax created opposition from Conservative members of the House of Commons, which induced Thatcher's resignation as prime minister and her replacement by John Major (Major, John), her chancellor of the Exchequer. These examples illustrate that “policy communities” in many different areas of government may command detailed expertise on issues that prime ministers may override—sometimes at their own peril. In most countries, premiers have tried to ease these limitations on their influence by building up their own policy staffs with expertise and establishing units to oversee the enactment of their key strategies and priorities. But there is only limited evidence that such policy centralization can work successfully.

Most countries in the world now have established an office of prime minister (alternatively called premier or chancellor). However, the United States and many African and Latin American countries have adopted presidential (president) systems consisting of an executive president (who is also head of state) and a separation of powers (powers, separation of) between the legislature and the executive. In these countries, the president picks his own cabinet or government, which does not depend on legislative support to remain in office (see presidency of the United States of America). Indeed, whereas in parliamentary systems prime ministers can be removed from office by the legislature through a simple vote of confidence, executive presidents can generally be removed from office only through a more cumbersome impeachment proceeding for serious crimes or abuse of office.

The strong prime minister model has been admired because it provides clear and decisive political leadership. In a somewhat different form it is found in several major European countries; for example, in Germany, Greece, Spain, and Sweden, premiers exercise considerable authority, even though the political systems of these countries are quite different from the Westminster model, operating with proportional elections and coalition governments. The key factor in such countries is that prime ministers lead large and integrated political parties and wield significant constitutional powers. Israel's prime ministers had been hampered in the late 20th century by shifting coalitions and a fractured party system, leading the Knesset (parliament) to adopt legislation in the mid-1990s that put in place the direct election of the prime minister. Intended to enhance the prime minister's position against parliamentary coalitions, this move backfired, and the new legislation in fact only further fragmented the party system; it was consequently soon rescinded.

Weak prime ministers are generally found in countries where coalition governments are the norm, involving several political parties that must work together to maintain a legislative majority (e.g., The Netherlands (Netherlands, The), Belgium, Switzerland, Austria, and Italy for most of the post-World War II period). Arguably, the Japanese (Japan) prime minister is also weak in comparison to the British prime minister, even though Japan generally has had single-party majority governments, led by the Liberal Democratic Party (Liberal-Democratic Party of Japan). In this model the prime minister is much less influential, especially when governments are short-lived or when they suffer from deep internal division (as was often the case in Italy until the 1990s). In coalition governments, for example, the premier generally has to balance representation among the various coalition party partners, doling out cabinet and junior ministerial appointments in numbers sufficient to ensure the support of all parties within the government. The premier often has little effective influence over who gets what ministerial role and may be unable to exert firm control over the direction of policy. With multiple parties sharing power the prime minister also does not have the same direct control over the legislative timetable and may even have to make executive decisions through consensus. In countries with weak prime ministers, department heads have more independent control of their ministries (particularly the finance and foreign ministers) and their own followings in parliament.

Even if the prime minister's party has a parliamentary majority (as in Italy in the 1950 and '60s or Japan for much of the post-World War II period), the prime minister's power may be reduced by the presence of strong internal factions, each headed by semi-independent leaders. In such cases, the prime minister may not be the uncontested—or even the primary—party leader; instead he may be a compromise figure able to appeal to voters and to hold the party factions together, perhaps only for a short period or until the government encounters a crisis or major problem. At other times the prime minister may not even come from the largest party in a coalition but may instead represent a smaller party with a central position in the majority coalition or a party whose involvement is necessary for the government to survive—as was often the case in Italy in the 1980s and '90s, when leaders such as Giovanni Spadolini of the Italian Republican Party and Bettino Craxi (Craxi, Bettino) and Giuliano Amato of the Italian Socialist Party held the prime ministership despite the political dominance of the Christian Democratic Party (see Italian Popular Party), their coalition partner. In weak prime minister systems, the premier may not be able to change the structure of government departments nor the operations of the civil service because of restrictive legal constraints—for example, changing the structure of ministries may require either new legislation (difficult to obtain if the government does not have a secure majority) or even a constitutional amendment. Sometimes the premier may be reduced to being no more than the “first among equals” within the executive branch, chairing cabinet discussions and having some power to set the government's agenda (especially over international summits) but not acting as a chief executive.

Prime ministers in semipresidential systems (e.g., France, Finland, Poland, and South Korea (Korea, South)) are a special case. In such systems, there is both a directly elected president with some substantial “reserved” executive powers and a presidentially appointed premier who must retain majority support in the parliament. If the president's party or coalition also controls a legislative majority (as in France under Pres. Charles de Gaulle (Gaulle, Charles de)), the prime minister is generally a secondary figure, responsible for the day-to-day running of the government and managing the president's legislative agenda. The president retains the authority to hire and fire the prime minister from among the leaders of his political party. The president is also preeminent in foreign and defense policy and in other strategic areas. The office of prime minister is much more important, however, in periods of “divided government,” when one party or coalition controls the presidency and a rival party or coalition retains majority support in parliament. During such periods, known as “cohabitation” in France, the president normally appoints the leader of the parliamentary majority as prime minister. The prime minister chairs the cabinet or government, which consists solely of ministers from his own party or coalition, and the premier's role resembles that of a company chief executive who has to get along with a powerful chairman. During cohabitation, the president's influence is often restricted to those issues where there is informal agreement between the president and prime minister and to any “reserved areas” where the constitution specifically requires the president's participation and agreement, usually foreign affairs, national defense, and major economic policy. The prime minister (like the relevant individual ministers) has to get along with the president, modifying policy enough to prevent a stalemate, despite the two leaders representing different parties. A successful prime minister in cohabitation governments is often viewed as a challenger to the president at the next election.

There is a final, ultraweak version of the office of prime minister, which occurs in countries with very strong presidents who have the political resources to influence electoral and legislative politics so as to “create” their own majority. In the 1990s Russian (Russia) Pres. Boris Yeltsin (Yeltsin, Boris) was able to secure approval for his prime ministerial appointees despite the fact that his backers did not hold a majority in the State Duma and a majority of representatives were often hostile to his government. His successor, Vladimir Putin (Putin, Vladimir), progressively exercised more power so as to define how parties compete in elections and how the Duma operates, thereby guaranteeing his government's legislative support and so reducing the role of the premier accordingly. Nonetheless, the premier remains constitutionally second in line to the president.

Within each model there are many variations over time and depending upon the different qualities of the people who occupy the office of prime minister. Skillful and charismatic politicians in systems that generally have weaker prime ministers may wield significantly more power and influence than their predecessors. And in normally strong prime minister systems, exceptional political conditions may occur (e.g., when the government is unable to command a majority or when the government is deeply unpopular) that reduce the usual power of the holder of the office. The prime minister's role in semipresidential systems often varies sharply depending on whether there is divided government or same-party control of the presidency and the premiership. Premiers may still exist in some presidential systems (e.g., Russia) where the constitutional powers vested in the presidency are vast.