"My Last Duchess" deals with the age-old theme of appearance versus reality. The Duke thought that his late wife had everything he could possibly want in a woman. Young, pretty, attractive, and with a lovely smile, she seemed like the perfect bride. One can imagine that, before he married her, the Duke would have seen a portrait of the Duchess. And from that portrait he will have inferred just what kind of person she was.

Unfortunately for him—and even more unfortunately for the Duchess—the appearance was very different from the reality. For although the Duchess did indeed have a beautiful smile in real life, she flashed it too freely at all around her, making her husband feel disrespected. The Duke felt cheated that the Duchess as she existed in real life was not the woman she appeared as in the portrait.

Unable to deal with the reality of the situation, the Duke took the drastic step of having the Duchess killed. And yet one can imagine that this psychotic tyrant will once more fail to distinguish between appearance and reality in his next choice of wife. This reinforces what we already know: that the problems with the Duke's last marriage were mainly on his side, not on that of the innocent Duchess's.

Robert Browning’s poem is both a character sketch of the duke and an artfully constructed narrative in poetic form. The author’s purpose is both to reveal the duke’s personality and actions and to achieve a carefully crafted work. The primary way that Browning achieves both of these objectives is by using a first-person narrator who is the duke. In this way, the poet places the words in the duke’s mouth, making it seem as though he is responsible for the information conveyed and the language used. Although the reader comes to understand that the duke is a ruthless tyrant who cares only about power, the duke himself is attempting to convince the person he is addressing of his accomplishments, including artistic patronage in hiring Fra Pandolf. He uses understatement and inference to convey both these things. For example, rather than state that he had his wife and the artist killed, he simply says “I gave commands.” As the reader is drawn in by the duke’s manner and words, they take the author’s hand for granted. In this way, the reader is convinced of the poet’s virtuosity without actually noticing it.

In "My Last Duchess," the Duke tells the story of his deceased wife, his "last" duchess, to an envoy who has been sent to negotiate another marriage for him. The Duke wants to raise sympathy for himself as he talks about his wife. However, the poem is ironic because the Duke, instead, conveys the message that he is a cruel narcissist who had an innocent young girl murdered for no good reason.

The poem conveys this message through the use of dramatic monologue. The Duke is showing the envoy around his palace and talking about the failings of the late Duchess. He discusses how she would blush and smile at other men, like the painter doing her portrait. He criticizes her for not having his refined tastes and for taking equal joy in such simple pleasures as eating cherries or riding around on her donkey as being in the presence of the Duke. He unintentionally shows that he is irrationally possessive and jealous. When he and the envoy stop and gaze at the portrait of the Duchess, it becomes clear that the Duke prefers her as an object he can possess and control, in the form of a painting.

Through the Duke's words, the envoy and the audience learn that the Duke would be a very dangerous person to marry. This is not the message he intends to convey, but it is the message he does convey.

Robert Browning’s main point in “My Last Duchess” is that the Duke values art but cannot appreciate beauty in reality. The truth is that reality contains infinitely more beauty than art, and no artist can hope to produce anything more than a crude copy. The Duke is incapable of expressing what it was about his late wife that displeased him, but it was the fact that, being real and being human, she lacked the professional polish and perfection that an expert artist can give to his copy.

Obviously, a beautiful young woman is far more of a “wonder,” more of a masterpiece of divine creation, than a two-dimensional copy of that same young woman in oil paints. But the Duke appreciates his copy more than he could ever appreciate the subject in real life. He sits there coveting and admiring his painting of his deceased wife, but he could only find fault with her when she was alive.

Fra Pandolf was well aware that his work at best was only a crude representation of reality. The Duke quotes him as saying, “Paint / Must never hope to reproduce the faint / Half-flush that dies along her throat.” The insensitive Duke thinks “Such stuff / was courtesy”—but the artist is telling the simple truth. Even the greatest artist must never hope to capture the subtle beauties of nature. Art is nothing but imitation. The artist can only remind us, or teach us, to appreciate the beauty that exists naturally all around us.

The Duke also prizes his sculpture of Neptune taming a sea-horse, but he would probably be blind to the ever-changing spectacle of the turbulent ocean which inspired the conception of the god Neptune and, many centuries later, Claus of Innsbruck’s cold, immobile bronze statue.