"Fair is foul and foul is fair."

--Act 1, Scene 1, Line 10: Part of the witches' conversation

This phrase is a metaphor that describes the state of affairs within Macbeth and without in Scotland.  Evil and sinister things have taken the place of all that is good and just.  Macbeth is a tyrannous ruler who consorts with witches and "murders" sleep; the kind and venerable King Duncan and Banquo are brutally killed.  In the midst of all of this, Inverness becomes a living hell for its inhabitants while Macbeth and his wife suffer from delusions and paranoia.

"Like valor's minion carved out his passage
Till he faced the slave."

--Act 1, Scene 1, Line 19: Description of Macbeth's courage in battle by the bloody captain

This metaphor, which likens Macbeth to "valor's minion," is ironic because whereas in this case his daring is advantageous, it is a curse later in the play as Macbeth relentlessly murders innocent subjects.

"And oftentimes, to win us to our harm,
The instruments of darkness tell us truths."

--Act 1, Scene 3, Lines 123-4: Banquo to Macbeth about the witches

The comparison of the witches to "instruments of darkness" reveals their truly foul nature.  Shakespeare is implying through Banquo that the honeyed prophecies of the weird sisters will only bring about Macbeth's downfall.  In addition, since Macbeth listens to the witches, he can be considered an "instrument of darkness" himself.

".that but this blow
Might be the be-all and end-all here,
But here, upon this bank and shoal of time,
We'd jump the life to come."

--Act 1, Scene 7, Lines 4-7: Macbeth to himself about King Duncan's impending death

Macbeth compares his indecision about killing Duncan to being on the bank of a river.  It is implied that this is the River Styx, the river that in Greek mythology that the damned had to cross over to enter hell.  Macbeth is thus likening his murderous thoughts to a damned soul.  He says that if it were sure that King Duncan's death would have no dire consequences, Macbeth would gladly "jump" (cross) the river (Styx) for the "life to come" (hell) in return for mortal pleasure.

".his virtues
Will plead like angels trumpet-tongued."

--Act 1, Scene 7, Lines 18-20: Macbeth to himself about King Duncan

By comparing King Duncan's virtues to "angels," Shakespeare implies that Macbeth has no virtues at all.

"The sleeping and the dead
Are but as pictures.  'Tis the eye of childhood
That fears a painted devil."

--Act 2, Scene 2, Lines 52-4: Lady Macbeth to her husband about killing those who are asleep

Lady Macbeth's comparison of the sleeping and the dead to "pictures" exemplifies her extraordinary courage and calm state of mind after the murder.  Lady Macbeth should supposedly be faint-hearted because she is a woman; in reality, however, she and her husband have switched roles.

"Who's there, in the name of Beelzebub?"

--Act 2, Scene 3, Line 4: The porter to himself

Although not technically a metaphor, this phrase is still important because the porter implies that Inverness is the dwelling-place of the devil himself.  Thus, Shakespeare implies that Inverness has both literally and figuratively become a living hell.

"Renown and grace is dead,
The wine of life is drawn, and the mere lees
Is left this vault to brag of."

--Act 2, Scene 3, Lines 93-5: Macbeth to himself about the status of his soul

In this metaphor, Macbeth compares his soul to an almost-empty wine bag.  Indeed, his heinous crime later renders him almost devoid of human emotion and compassion.

"Upon my head they placed a fruitless crown
And put a barren scepter in my grip,
Thence to be wrenched with an unlineal hand,
No son of mine succeeding."

--Act 3, Scene 1, Lines 61-4: Macbeth about the witches' prophecies

Macbeth laments that although the witches prophesized that he would become king, they also said that Banquo's posterity would possess the throne as well.  His jealousy from this statement induces him to kill Banquo and attempt to kill Fleance.

"There the grown serpent lies; the worm that's
fled
Hath nature that in time will venom breed,
No teeth for th'present."

--Act 3, Scene 4, Lines 28-31: Macbeth to himself about Banquo and Fleance

Macbeth likens the dead Banquo to a deceased serpent and his son Fleance to a young snake.  This metaphor is important because it implies that Macbeth still considers Fleance a threat even though Banquo is dead.

"Hold fast the mortal sword, and like good men
Bestride our down-fall'n birthdom."

--Act 4, Scene 3, Lines 3-4: Macduff to Malcolm about saving Scotland

In this metaphor, Macduff compares courage to a "mortal sword." This is important because it portrays Macduff's willingness to fight for his country.

"Angels are bright still, though the brightest fell:
Though all things foul would wear the brows of grace."

--Act 4, Scene 3, Lines 22-3

Malcolm compares Macbeth to a fallen angel, thus implying that he is Lucifer himself.

"I think our country sinks beneath the yoke;
It weeps, it bleeds, and each new day a gash
Is added to her wounds."

--Act 4, Scene 3, Lines 39-41: Macduff to Malcolm

Macduff draws a parallel between Scotland and a beast of burden.  Like an abused animal, Scotland is on the verge of collapsing underneath its tyrannous master (Macbeth).

"Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage
And then is heard no more.  It is a tale
Told by an idiot, full of sound and fury,
Signifying nothing."

--Act 5, Scene 5, Lines 24-9: Macbeth to himself after his wife's suicide

In this world-renowned quote, Macbeth compares life to an ineffectual actor.  This metaphor is important because it exemplifies his fatalistic and nihilist tendencies as well as his apathy for his wife's death.