The Leader of the Opposition is a title traditionally held by the leader of the largest party not in government in a Westminster System of parliamentary government. The Leader of the Opposition is often seen as the alternative Prime Minister, Premier or Chief Minister to the present incumbent, and heads a rival alternative government known as the Shadow Cabinet or Opposition Front Bench.

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

For information about the leader of the opposition in a particular parliament, see the following: