Although the United States constitution provides for some checks, Donald Trump's power as president will be immense. The president commands the armed forces, with the power to direct troops, has primary responsibility for foreign policy and is entitled to withhold information from Congress, the courts and the public in the interests of national security.
He can veto any bill passed by Congress, and his veto can only be overridden by a two-thirds vote in both the Senate and the House of Representatives. He can pardon or offer clemency to criminals, a potentially useful power for Trump.
Trump will also prepare the federal budget, although it must be approved by Congress, and he can influence how federal agencies operate by issuing new rules, regulations and executive orders.
Before he takes office Trump will appoint more than 6,000 federal officials, including the top five layers of the civil service. Some of his appointments, such as ambassadors and federal judges, will require the advice and consent of the Senate, where Republicans on Tuesday retained their majority.
Appointments
Perhaps his most important appointments will be to the supreme court where justices serve for life. There is already one vacancy and the ages of some of the justices mean that more are possible. The supreme court can overrule a presidential decision to withhold information, and executive orders can be subject to judicial review.
Congress could impeach Trump for treason, bribery or other high crimes and misdemeanours but it is a long politically charged process. The House of Representatives must first decide there are grounds for impeachment, after which the Senate puts him on trial. Even if he is convicted, he will not necessarily have to leave office.