Nietzsche contra Wagner; Out of the Files of a Psychologist is a critical essay by Friedrich Nietzsche, composed of selections he chose from among his earlier works. The selections are assembled in this essay in order to focus on Nietzsche's thoughts about the composer Richard Wagner. As he says in the preface, when the selections are read "one after the other they will leave no doubt either about Richard Wagner or about myself: we are antipodes." He also describes it as "an essay for psychologists, but not for Germans". It was written in his last year of lucidity (1888–1889), and published by C. G. Naumann in Leipzig in 1889. Nietzsche describes in this short work why he parted ways with his one-time idol and friend, Richard Wagner. Nietzsche attacks Wagner's views, expressing disappointment and frustration in Wagner's life choices (such as Nietzsche's mistaken belief that Wagner had converted to Christianity, perceived as a sign of weakness). Nietzsche evaluates Wagner's philosophy on tonality, music and art; he admires Wagner's power to emote and express himself, but largely disdains what the philosopher deems his religious biases.

It is easy to suspect that Nietzsche's views must be motivated by a personal quarrel with Wagner. However, Nietzsche had nothing to gain by attacking Wagner, his motives were misunderstood by a public who were influenced by Nietzsche’s early admiration of Wagner, and who were now enthralled by Wagner’s genius. These essays would be hard to comprehend, and would be seen as the work of a disloyal fanatic. The attacks also at times, confusingly pause to express an affectionate appreciation for Wagner.

According to Roger Hollinrake, it is reasonable to question Nietzsche’s qualifications to criticize a great musician on very specific musical topics. Nietzsche was a philosopher, and also a musician and composer, though of limited abilities. However gifts of analysis, and gifts of musicianship are not often both given to any one individual. Nietzsche had the broad combined perspective of a scholar, philosopher, historian and poet, abetted by his penetrating insight and an objectivity with a lack of musical bias. Nietzsche's attacks derive from the great importance he gives to art, and from his sense of the seriousness of the artist's duties, and from Nietzsche's fears for the state of culture in his era.

The sections are as follows. Nietzsche’s sources, found among his works, are indicated in parentheses:

Nietzsche explains that this book consists of selections from his previous writings. They show that he and Wagner are opposites. Nietzsche states that a reader of this book will conclude that it is for psychologists and not for Germans. He says that he has readers almost everywhere, in New York and in Europe, but not in Germany.

(cf. The Gay Science, 87)
Nietzsche praises Wagner in a number of ways, saying that Wagner

is a master at finding tones in the realm of human suffering, depressed and tortured souls, at giving voice to even mute misery. None can equal him in the colors of late fall, in the indescribably moving happiness of the last, truly last, truly shortest joy.

Nietzsche continues, and then adds that Wagner's music has revealed "some very minute and microscopic aspects of the soul … indeed he is the master of the very minute. But he does not want to be that!" Nietzsche suggests with a metaphor that Wagner prefers to create large works: "His character prefers large walls and audacious frescoes."