The Leader of the Opposition is a title traditionally held by the leader of the largest opposition party in a Westminster System of parliamentary government. The Leader of the Opposition is invariably seen as the alternative Prime Minister to the present incumbent, and heads a rival alternative government known as the Shadow Cabinet or Opposition Front Bench.

In many Commonwealth Realm monarchies, the full title is Leader of Her Majesty's Loyal Opposition.

See also:

• Leader of the Opposition (Australia)
• Leader of the Opposition (Canada)

• Leader of the Opposition (Ontario)

• Leader of the Opposition (India)
• Leader of the Opposition (Ireland)
• Leader of the Opposition (New Zealand)
• Leader of the Opposition (UK)

• Official opposition
• Loyal opposition