Identity
Identity is a significant theme in this play. For nearly the entire play, Menaechmus of Syracuse is startled by everyone in Epidamnus knowing who he is, while he knows no one. In fact, Messenio warns him that the city is full of people that cannot be trusted, and everyone knowing who he is only proves that. Throughout the play, characters interact with one another without fully knowing who they are speaking with, even though they think they know. Both Erotium and Menaechmus of Epidamnus’ wife think that they are speaking with Menaechmus of Epidamnus during the play, when they are in fact speaking with his twin. The fact that they think he is Menaechmus of Epidamnus, who is just behaving strangely, leads them to believe that he is mad.
Mythology
Throughout the play, the Trojan war is frequently referenced by the characters. By referencing the Trojan War, Plautus keeps his content relevant to his contemporary audience and provides a break from the madness happening on stage in Epidamnus. Mythology is also referenced when Menaechmus of Syracuse is pretending to have gone mad, as he tells Menaechmus of Epidamnus’ wife and father-in-law that Dionysus—the god of ritual madness—is telling him to gouge the wife’s eyes out.
Slavery
Slavery is a theme throughout the play, with each character being a slave to something or someone. Most obviously, Messenio is Menaechmus of Syracuse’s slave. However, other characters are enslaved as well. Peniculus, for example, is a slave to his appetite, doing whatever it takes to ensure he gets to feast. Indeed, sating his appetite is his only concern. Menaechmus of Epidamnus is a slave to his social obligations and responsibilities. Menaechmi of Syracuse is enslaved by his search for his brother (Messenio complains at the beginning of the play that he and Menaechmus of Syracuse have been searching for his brother for six years).
Madness
Due to the mistaken identity caused by the brothers' presence in the same town, there are numerous incidents in which people claim that someone is "mad." For Menaechmus II, the people in this town who pretend to know him and accuse him of a variety of things are mad. For Menaechmus I, all the people he associates with in otherwise normal ways are now seemingly mad. The most conspicuous example of madness, or perceived madness, is when Menaechmus II decides the only way he can rid himself of these annoying people is to feign madness, which leads the Father and the Doctor to definitively claim the young man is insane and needs treatment. Madness means that a person cannot operate properly in society and is a danger to it and to himself. Though little understood in terms of its origins, madness was definitely understood as something that was not tolerable in society.
Pleasure vs. Obligation
The Wife and Erotium represent the two poles of a Greek man's life: obligation and pleasure. The Wife constrains and represses Menaechmus, holding him back from the pleasures he wishes to indulge. Related to that, Menaechmus I also has to attend to his business at the forum, a tedious situation that vexes him. By contrast, Erotium and her offerings of sex, praise, and good food and drink are everything that Menaechmus I desires but cannot have as frequently as he wants. When he balances obligation and pleasure, life is generally tolerable and harmonious, but if he behaves out of line (e.g., stealing his wife's dress), then the balance is threatened.
Gender
The Greeks/Romans seem to have pretty recognizable views on gender. Woman is either a beguiling courtesan (Erotium) or a carping Wife, intent on preventing her husband from indulging in the pleasures he is putatively due. Men are the public figures, going out to work or socialize with friends or travel the world. Men are the authority within the home, but depending on their social station, they have cues to follow and roles to play in society at large. They must meet their obligations, but they have a much larger degree of autonomy.
Social Class
There are varying levels of social class in the text: the well-to-do, as seen in the Menaechmus brothers; the paid woman, Erotium; the slave, Messenio; the parasite, Brush; and the professional, the Doctor. Social class dictates what a person can do, where they can go, how they can act, etc., but it also cannot account for intellect and cleverness. Brush and Messenio are two of the cleverest characters in the play though they are at the lowest rungs of the social ladder.