Outline
The movement is radical in its harmonic outline, building a vast arch back to the home key, which, while similar to the Sonata Form of classical composition, was taken as a departure by Parisian critics. It is here that the listener is introduced to the theme of the artist's beloved, or the idée fixe. Throughout the movement, there is a simplicity of presentation of the melody and themes, which Schumann compared to "Beethoven's epigrams", ideas which could be extended, had the composer chosen to. In part, it is because Berlioz rejected writing the very symmetrical melodies then in academic fashion, and instead looked for melodies which were, "so intense in every note, as to defy normal harmonization", as Schumann put it.
The theme itself was taken from Berlioz's scène lyrique "Herminie", composed in 1828.

First movement: "Rêveries - Passions"
The second movement takes a rather plain waltz theme, again, derived from the idée fixe at first, and then transforming it. It is filled with running ascending and descending figures. While one critic called it "vulgar", the intent was to portray a single lonely soul amidst gaiety, as Berlioz wrote while composing it.

Second movement: "Un bal"
The third movement opens with the English horn and offstage oboe tossing back and forth a characteristic melody meant to evoke the horns in the mountains. The English horn represents the artist and the oboe his beloved. The melodies of these instruments represent the artist and his beloved calling back-and-forth. This intent, to evoke a spirit of the country side inhabited by, not mere rustics, but people who were one with their place is part of Romanticism and can be traced back to the ideas of such writers as Goethe. The idée fixe comes back. The movement swells to a peak, as if the artist is pushing away the idea of his beloved, the dramatic sounds fall away. The sound of distant thunder comes, in an innovative passage for four timpani players on two sets of timpani: it ends without resolution.

Third movement: "Scène aux champs"
The fourth movement, which Berlioz claimed to have written in a single night (but which he actually took from an unfinished project, the opera Les Francs-juges), is filled with blaring horns and rushing passages, and scurrying figures which would later show up again in the last movement. The movement describes a dream, in which the artist is executed for killing the love of his life. It uses a grotesque version of the theme by Berlioz's extraordinary technique of orchestration, mixing string pizzicato, woodwind staccato, brass chords and a single loud stroke of percussion, forming a highly unusual series of tone colors. The scene ends with a single short fortissimo G-minor chord that represents the fatal blow: the dropping of the trap door, or perhaps the guillotine blade; the series of pizzicato notes following can be seen to represent the rolling of the severed head into the basket. Immediately prior to the musical depiction of the beheading, there is a brief, nostalgic recollection of the idée fixe in a solo clarinet, as though representing the last conscious thought of the executed man; after his death, the final nine bars of the movement contain a victorious series of tutti G major chords, seemingly intended to convey the cheering of the onlooking throng.

Fourth movement: "Marche au supplice"
The last movement, often played as a tone poem by itself, has a brooding opening, the sound of spirits marching through the graveyard. There follows, in turn, a familiar E-flat clarinet solo presenting the idée fixe as a vulgar dance tune; the call of church bells; a burlesque of a famous plainchant, the Dies Irae; and a fugue meant to represent, as Berlioz privately admitted, a giant orgy. There are a host of effects (including eerie col legno playing in the strings), from the bubbling of the witches' cauldron to the blasts of wind. The climactic finale of the symphony combines the somber Dies Irae melody with the wild fugue of the Ronde du Sabbat (Sabbath Round).

Fifth movement: "Songe d'une nuit de sabbat"
Berlioz wrote in his essay "On Imitation in Music":
The aim of the second kind of imitation, as we have said before, is to reproduce the intonations of the passions and the emotions, and even to trace a musical image, or metaphor, of objects that can only be seen.
He later adds:
emotional (imitation) is designed to arouse in us by means of sound the notion of the several passions of the heart, and to awaken solely through the sense of hearing the impressions that human beings experience only through the other senses. Such is the goal of expression, depiction or musical metaphors.
As part of this he uses an example of cyclical structure in music, which was an idea drawn from Beethoven's use of similar rhythmic structures or shapes, and the idea of musical "cycles", such as a "song cycle". Berlioz did not know of Mendelssohn's Octet, which uses this device as well.
Leonard Bernstein called this symphony the first musical expedition into psychedelia because of its hallucinatory and dream-like nature, and because history suggests Berlioz composed at least a portion of it under the influence of opium.
In 1831, Berlioz wrote a much less well known sequel to the work, Lelio, for narrator and orchestra.

Instrumentation
Berlioz fell in love with an Irish actress, Harriet Smithson, after attending a performance of Shakespeare's Hamlet with her in the role of Ophelia, on 11 September 1827. He sent her numerous love letters, all of which went unanswered. When she left Paris they had still not met. He then wrote the symphony as a way to express his unrequited love. It premiered in Paris on December 5, 1830; Harriet was not present. She eventually heard the work in 1832 and realized that she was the genesis. The two finally met and were married on October 3, 1833. While the marriage was happy for several years, they were divorced nine years later, partially due to the language barrier between them.