When published in 1902 in a volume with two other stories (Youth and The End of the Tether), Heart of Darkness was praised for its portrayal of the demoralizing effect life in the African wilderness supposedly had on European men. One respected critic of the time, Hugh Clifford, said in the Spectator that others before Conrad had written of the European's decline in a ‘‘barbaric’’ wilderness, but never ‘‘has any writer till now succeeded in bringing … it all home to sheltered folk as does Mr. Conrad in this wonderful, this magnificent, this terrible study.’’ Another early reviewer, as quoted in Leonard Dean's Joseph Conrad's ‘Heart of Darkness’: Backgrounds and Criticisms, called the prose ‘‘brilliant’’ but the story ‘‘unconvincing.’’

In his review published in Academy and Literature in 1902, Edward Garnett called the volume's publication ‘‘one of the events of the literary year.’’ Garnett said when he first read Heart of Darkness in serial form, he thought Conrad had ‘‘here and there, lost his way.’’ But upon publication of the novel in book form, he retracted that opinion and now held it ‘‘to be the high-water mark of the author's talent.’’ Garnett went on to call Heart of Darkness a book that ‘‘enriches English literature’’ and a ‘‘psychological masterpiece.’’ Garnett was particularly taken with Conrad's keen observations of the collapse of the white man's morality when he is released from the restraints of European law and order and set down in the heart of Africa, given free reign to trade for profit with the natives. For sheer excitement, Garnett compared Heart of Darkness favorably to Crime and Punishment, published by the great Russian novelist Dostoyevsky in 1866. Garnett calls Heart of Darkness ‘‘simply a piece of art, fascinating and remorseless.’’

Kingsley Widmer noted in Concise Dictionary of British Literary Biography that Conrad's literary reputation declined sharply in the mid-1920s after the publication of Victory, which Widmer flatly called a ‘‘bad novel.’’ But the following generation gave rise to a revival of interest in Conrad's work, centering largely on a few works written between 1898 and 1910 and including Heart of Darkness, The Secret Agent, and Lord Jim, which were given the status of modern classics.

Widmer concluded that although ‘‘much of Conrad's fiction is patently poor,’’ his sea stories contain a ‘‘documentary fascination in their reports of dying nineteenth-century merchant marine sailing experience.’’ Widmer faults Conrad for gross sentimentality, shoddy melodrama, and chauvinism. But he acknowledges that Conrad's best fiction, among which he counts Heart of Darkness, Nostromo, The Secret Sharer, and The Secret Agent, which he says may be ‘‘Conrad's most powerful novel,’’ achieves a modernism that undercuts those heavy-handed Victorian characteristics and provides the basis on which Conrad's reputation justifiably rests.

In more recent years, Heart of Darkness has come under fire for the blatantly racist attitudes it portrays. Some critics have taken issue with the matter-of-fact tone in which Marlow describes Africans as ‘‘savages’’ and ‘‘niggers’’ and portrays African life as mysterious and inhuman. Noted Nigerian author Chinua Achebe for instance, argued in a Massachusetts Review article that ‘‘the question is whether a novel which celebrates this dehumanization, which depersonalizes a portion of the human race, can be called a great work of art. My answer is: No, it cannot.’’ Other critics, however, have reasoned that Conrad was merely portraying the views and attitudes of his time, and others have even suggested that by presenting racist attitudes the author was ironically holding them up for ridicule and criticism.

Despite such controversy, Heart of Darkness has withstood the test of time and has come to be seen as one of Conrad's finest works. The way in which Conrad presents themes of moral ambiguity in this novel, never taking a side but forcing the reader to decide the issues for him or herself, is considered a forerunner of modern literary technique. Frederick Karl, in Joseph Conrad: The Three Lives, calls Heart of Darkness the work in which ‘‘the nineteenth century becomes the twentieth.’’ Others have called it the best short novel in the English language. ‘‘The Secret Sharer and Heart of Darkness,’’ said Albert J. Guerard in his introduction to the novel, ‘‘are among the finest of Conrad's short novels, and among the half-dozen greatest short novels in the English language.’’ The book continues to this day to be taught in high schools, colleges, and universities and to be held up as an example of great literature.