William Shakespeare took his inspiration from many areas, including the Bible, nature, other writers and historians. Two of Shakespeare's favorite writers were Geoffrey Chaucer and Plutarch; their works would influence many of his plays.
Geoffrey Chaucer is considered one of the major poets of England. His poem, "Troilus and Criseyde," was Shakespeare's main source for his play, "Troilus and Cressida." Chaucer's influence can also be seen in Romeo and Juliet, where Mercurio's speech is based on "Parliament of Fowles."
Plutarch was an ancient Roman historian and philosopher. His biography, "Parallel Lives," was used as a source for many of Shakespeare's history plays, including "Julius Caesar," "Antony and Cleopatra" and "Timon of Athens."