The extended projection principle (EPP) is a linguistic hypothesis about subjects.
It was proposed by Noam Chomsky as an addendum to the projection principle.
As a result, verbs that do not assign external theta roles will appear with subjects that are either dummy pronouns (e.g. expletive "it," "there"), or ones which have been moved into subject position from a lower position (e.g., subject of an embedded clause after the verbs, like seem, appear etc.
Examples proposed to be the result of expletive subject insertion in accordance with the EPP: Notice that in all of these the overt subject has no referential reading.
In languages that allow pro-drop (such as Spanish or Italian), the empty category pro (not to be confused with Big PRO) can fulfill the requirement of the EPP.