The radio series is comprised of six specific "phases":
Primary and Secondary Phases
The series was originally written as a radio serial (rather than being adapted from a book), and was conceived and realised by Douglas Adams. What is now known as the Primary Phase was the first part to be recorded in 1978 and broadcast in March and April 1978. Five further episodes were broadcast in January 1980, and, along with Fit the Seventh, which had previously been a Christmas special broadcast in 1978, these six episodes form the second part of the original series; the Secondary Phase. These first two "phases" were later adapted into the books The Hitchhiker's Guide to the Galaxy and The Restaurant at the End of the Universe, though there are notable story differences between the books and radio series.
Tertiary, Quandary and Quintessential Phases
Three more radio series, the Tertiary Phase, Quandary Phase and Quintessential Phase, were broadcast in 2004 and 2005. These radio series were commissioned and produced 23 years after the original series had ended, and so contained several cast and crew changes. They also came after the death of Douglas Adams in 2001, and Dirk Maggs, John Langdon and Bruce Hyman adapted and directed these series following instructions left by Adams. These three phases are adapted from the third, fourth and fifth books in the Hitchhiker's series: Life, the Universe and Everything, So Long, and Thanks For All the Fish, and Mostly Harmless.
- See also Hexagonal Phase
After another long hiatus, the final radio series was produced in 2018, and was broadcast on Thursday 8 March 2018, 40 years to the day of the first series' original broadcast in 1978. It was based on the sixth novel in the series, And Another Thing..., which was written by Eoin Colfer after Adams' death. It also contains additional unpublished material by Douglas Adams.