The structure of the novel is both complicated and interesting. Stevenson splits the novel into ten chapters each given a title:
The titles show us how tension is built in the novel towards a climax in chapters 4-8.
The complex structure is, in part, because of the number of different narrators. Utterson narrates the bulk of the novel before we get Lanyon's account and then finally Jekyll's. This must be the case because if we heard from Jekyll before this then the mystery would be revealed.