Crime
Crime is a major theme in the novel, it is used to show the characters desperation and the lengths they will go to so they can buy drugs. The characters steal money and also steal prescription drugs from old people in desperation.
Heroin
Heroin is what the novel centers around, to the extent that it's almost a character in itself; as Sick Boy said, "heroin has a great personality." In the novel although at some points it seems Welsh is glamourizing the drug, it is ultimately depicted as the root of all evil
Music
Music plays a considerable part in the novel. Renton is obsessed with Iggy Pop, for instance. A lot of the time, the music is closely related to the drug taking; whenever the characters are high, they are usually listening to music
