Jealousy is a very prominent theme in Othello, driving the actions of some of its main characters. One instance of jealousy occurs very early in the play. Iago, who deems himself a very able officer, has been passed over for a promotion by Othello, who has chosen Michael Cassio to serve as his lieutenant. The whole affair, Iago says, has been unfair from the start, as Iago says:
'Tis the curse of service,
Preferment goes by letter and affection,
And not by old gradation, where each second
Stood heir to the first. Now, sir, be judge yourself
Whether I in any just term am affined
To love the Moor.
This incident, it seems, is what motivates Iago to launch his deadly scheme against the Moor. Fittingly, Iago's plan plays on Othello's jealousy, which he sparks by contriving to make Othello believe that his wife Desdemona has been unfaithful to him. So the other obvious example of jealousy comes at the end of the play, when Iago's plan is consummated with Othello's murder of his innocent wife, who dies (rightly) asserting her innocence. Othello, believing that his wife has betrayed him, is unmoved:
By heaven, I saw my handkerchief in ‘s hand.
O perjured woman! Thou dost stone my heart,
And makest me call what I intend to do
A murder, which I thought a sacrifice:
I saw the handkerchief.
Iago's jealousy sets the plot in motion, and the genius of the scheme is the skill with which he plays on the jealousies of Othello.