Light Thickens is a detective novel by Ngaio Marsh; it is the thirty-second, and final, novel to feature Roderick Alleyn, and was first published in 1982. The plot concerns the murder of the lead actor in a production of Macbeth in London, and the novel takes its title from a line in the play. A number of characters in the book appeared previously in Marsh's novel Death at the Dolphin. The novel is dedicated to the actors James Laurenson and Helen Thomas who had played Macbeth and Lady Macbeth respectively in the author's 1962 production of the play, which she had previously directed, also for The Canterbury University Players, in 1946.