Dom Casmurro is considered by critic Afrânio Coutinho "a true Brazilian masterpiece, and perhaps Brazil's greatest representative piece of writing" and "one of the best books ever written in the Portuguese language, if not the best one to date."

Credited as a forerunner of Modernism[3] and of ideas later written by the father of psychoanalysis Sigmund Freud,[4]: 144  the book influenced writers such as John Barth, Graciliano Ramos and Dalton Trevisan, and is considered by some to be Machado's masterpiece, on a par with The Posthumous Memoirs of Brás Cubas.

In the first chapter, the author explains the title: it's a tribute to a "train poet" who once pestered him with his verses and called him "Dom Casmurro" because, according to Bento, he "closed his eyes three or four times" during the recitation.

[4]: 163 After The Posthumous Memoirs of Brás Cubas (1881), Machado de Assis wrote books with different themes and styles from his earlier novels, such as Resurrection and The Hand and the Glove.

"[4]: 134  Ian Watt has stated that realism refers to the empirical experiences of men,[10]: 12  but the recreation of the past through Bentinho's memory, his "stains" of recollections, brings the book closer to an impressionist novel.

"[11]: 14  We can therefore conclude that Dom Casmurro is a realist novel that focuses on psychological analysis (or exposition) and ironically criticises society, in this case the elite of Rio de Janeiro, through the behaviour of certain characters.

Some, such as Roberto Schwarz [pt], even go so far as to call it "the first Brazilian modernist novel",[3] mainly because of its short chapters, its fragmentary, non-linear structure, its penchant for the elliptical and the allusive, the metalinguistic attitude of those who write and those who see themselves as writers, the interruption of the narrative and the possibility of multiple readings or interpretations; "anti-literary" elements that would only be popularised by modernism decades later.

"[2]: 116  According to Roberto Schwarz, in Dom Casmurro "the most dramatic instance lies in jealousy, which had been one of the boy's many imaginative outbursts, and now, associated with the authority of being the landowner and husband, it becomes a force for devastation".

[20]: 1081  The critic Barreto Filho, for example, noted that it was "the tragic spirit that would shape Machado's entire work, leading destinies towards madness, absurdity and, in the best of cases, solitary old age.

[2]: 221  As a lawyer, Bento also makes use of rhetoric to present his version of the facts;[23]: 192  his narrative, in psychological time, follows the shifts of his memory in a less random way than in The Posthumous Memoirs of Brás Cubas, but just as fragmentarily.

[4]: 163  However, some themes are noticeable: his childhood in Matacavalos; Dona Glória's house and the Pádua family, relatives and households; his acquaintance with Capitu; the seminary; married life; the intensification and outbursts of jealousy; the separation, etc.

[4]: 163 In fact, the book's style is very close to that of associative impressionism, with a break in the linear narrative, so that the actions do not follow a logical or chronological thread, but are told as they emerge from Bento Santiago's memory and will.

[9]: 84  José Guilherme Merquior noted that the style of the book "remains in line with the two previous novels, with short chapters, marked by appeals to the reader in a more or less humorous tone and by digressions between seriousness and humour".

[28]: 236  For American critic Helen Caldwell, this quote is the one that sets Bento's memory in motion: "closely followed by the allegory of the 'opera', with its colloquies in heaven between God and Satan, gives the impression that Santiago perhaps identified himself with Faust and felt he had sold his soul to the devil".

[29]: 131  Critics note that the ancient and modern classics and the biblical quotations are never mere erudition in Machado; on the contrary, they enlighten the narratives and properly inscribe them in the great archetypes of universal literature.

[29]: 3-4 Other sources refer to the physical resemblance of the son (in this case, Ezequiel) as a result of the mother being "impregnated" with the features of a beloved man, without the latter having conceived him, a theme already used earlier by Zola in his Madeleine Férat (1868) and also in Goethe's earlier The Elective Affinities (1809), where Eduard and Charlotte's son has the eyes of Ottilie, with whom Eduard is in love, and the features of the captain, loved by Charlotte;[22]: 181  Bento's pessimistic philosophy, where critics critics have noted the direct influence of Schopenhauer, for whom "the pleasure of existence does not rest in living, but is only achieved in contemplating what has been lived" (hence Bento's aim to portray his past),[22]: 182 [11]: 148  and Pascal, as Bentinho's Christianity is analogous to the Jesuit casuistry attacked by him and the Jansenists.

[34]: 82 Bentinho's challenge to attract and win Capitu over to achieve his goal of being interested in his girlfriend's gifts – an attitude identified as a "lordly and possessive model that dispenses with greater subtleties" – indirectly influenced Graciliano Ramos when he wrote one of his most famous modernist novels of the 20th century, São Bernardo [pt] (1934), portraying Paulo Honório's direct action in capturing Madalena.

[4]: 144 The subsequent adaptations of Dom Casmurro, in countless media and forms, also prove the dialogue and influence that the novel still has in many different areas, whether in cinema, theatre, popular and classical music, television, comics, literature itself, etc.

Dom Casmurro has also been analysed in studies of sexuality and the human psyche, and existentialism, so that in recent times Machado's work has been commonly attributed an open range of interpretations.

[14]: 228  The majority of interpretations of the novel are influenced by sociology, feminism and psychoanalysis,[6]: 7  and most also refer to the theme of the narrator's jealousy, Dom Casmurro; some arguing that there was no adultery and others that the author left the question open to the reader.

[23]: 190  Bentinho is seen as a typical 19th century Brazilian man from Rio's high society, with no historical perspective (hence his desire to write History of the Surburbs but then choosing to first recount the memories of his youth), pessimistic and elusive.

[41] The interpretative criticism negatively directed at the narrator and Capitu's salvation, arguing that she didn't betray him, has only recently been made, since the feminist movement of the 1960s and 1970s and above all by the American essayist Helen Caldwell.

In The Brazilian Othello of Machado de Assis (1960), she argues that the character Capitu did not betray Bentinho, thus changing the prevailing perception of the novel,[11]: 7  and that she is the victim of a cynical Dom Casmurro who actually misleads the reader with words that are not true.

For Dona Glória's carrion, mouldy and repressed universe, with its widowers, servants and slaves, the energy and freedom of opinion of the modern, poor girl, daring and irreverent, lucid and active, would become intolerable.

[4]: 130  One of the evidences of Gledson's argument can be found in chapter 3, which he considers to be the "foundation of the novel", in José Dias' motivation when talking about Capitu's family and reminding Mrs Glória of the promise she made to put Bentinho in the seminary, in other words, treating the "Pádua people" as inferior and their daughter as a dissimulated and poor girl who could corrupt the boy.

[11]: 26, 165 Thus, the jealousy of Bentinho, a rich boy from a decaying family, of the typical bachelor of the Second Empire, would condense a broad social problem behind this "new Othello who slanders and destroys his beloved".

[4]: 130  This sociological interpretation is still preserved today, as we read in the words of the Portuguese essayist Hélder Macedo who made a statement regarding the theme of jealousy: From this perspective, the narrator, a stereotypical tool used by the author to criticise a certain social class of his time,[4]: 130  is able to use the prejudices of Brazilians to induce them in his argument against Capitu.

[49] José Veríssimo wrote that "Dom Casmurro is about an undoubtedly intelligent man, but a simple one, who from an early age let himself be deceived by the girl he had loved as a child, who had bewitched him with her calculated cheekiness, with her deep innate science of dissimulation, to whom he had given himself with all the fervour compatible with his quiet temperament".

[3] As we know, however, Livraria Garnier published Machado's volumes both in Brazil and in Paris, and with the new book, international critics were already questioning whether Eça de Queiroz was still the best Portuguese language novelist.

[5]: 13  Before these two productions, Dom Casmurro was adapted into an opera, with a libretto by Orlando Codá and music by Ronaldo Miranda, which premiered at the Theatro Municipal de São Paulo in 1992.