Doctor Who: The Monthly Adventures, formerly subtitled as the Main Range, is a series that consists of full-cast audio dramas based on the British science fiction television programme Doctor Who. They were produced by Big Finish Productions between 1999 and 2021, and star one or more of the original actors who played the Doctor on television in the classic era of the programme.

Doctor Who revolves around the adventures of an alien called the Doctor, whose people are called the Time Lords. He travels in space and time using a spaceship called the TARDIS, which can travel in time as well as space. While travelling, the Doctor works to save lives and liberate oppressed peoples by combating foes. The Doctor often travels with companions. The programme has a concept of regeneration, in which, when a Time Lord is fatally injured, their cells regenerate and they are reincarnated into a different body with a different personality, but the same memories.^{[citation needed]}

The Classic Series of Doctor Who which these stories are based on originally ran from 1963 to 1989 with a TV movie in 1996. In 1999, Big Finish obtained a non-exclusive licence to produce official Doctor Who audio plays.

Big Finish Productions began producing audio dramas featuring the Fifth Doctor, Sixth Doctor, and Seventh Doctors, under the "Main Range" banner, starting with the multi-Doctor story The Sirens of Time in July 1999. They gradually developed a pattern of thirteen releases per year, one every month with two in September or December. From 2001 to 2007, the main range also included releases featuring the Eighth Doctor; future releases with the Eighth Doctor were more sporadic, coincident with the beginning of the Eighth Doctor Adventures. Various companions appeared throughout the adventures in main and guest roles, mostly originating from television, though some originated from various spin-offs such as novels and comics, and some debuted in the Big Finish audios themselves.

In May 2020, Big Finish announced that The Monthly Adventures (which had recently changed its subtitle) would conclude with its 275th release, to be replaced with regular releases of each Doctor in their own boxsets throughout the year from January 2022, as part of a revamp. The number of these boxsets for each Doctor was announced in May 2022, with the first, second and fifth getting one boxset, and the other five, including the third and fourth, receiving two. With 275 releases over 22 years, the series holds the Guinness World Record for longest running science fiction audio play series.

Though the programme had been put on indefinite hiatus in 1989 (and despite the failed revival attempt with the 1996 TV movie), the BBC still published, as well as gave non-exclusive licenses to other companies to produce Doctor Who stories through various mediums such as novels and comics. Big Finish Productions, which mostly consisted of fans who started out recording fan audio plays, were given the license to record some of the New Adventures, a series of novels from Virgin Books which had originally been licensed Doctor Who stories, into audio plays. This eventually transitioned into a license to produce original audio dramas featuring the past Doctors, with The Sirens of Time being the first story to be released in July 1999.

A number of Doctor Who spin-off writers formed the group of original writers, including Gary Russell, Nicholas Briggs, Justin Richards and Mark Gatiss. Future Doctor Who showrunner Steven Moffat also joined, but left after it became clear that the license did not extend to the then-incumbent Eighth Doctor. The still alive actors of the Fourth to Eighth Doctors were approached for the role, but Tom Baker, the Fourth Doctor, declined. The releases therefore alternated between the Fifth, Sixth and Seventh Doctors, with the license extended to include the Eighth Doctor in 2001, although his releases generally weren't included in the alternating schedule, instead that years 8th doctor releases would all come out one after the other before then returning to the normal rotation of the other doctors. Doctor Who was revived in 2005, returning to television, and the next few years saw the introduction of new spin-offs, including The Eighth Doctor Adventures, and the Eighth Doctor mostly moved away from the Main Range.

Various companions from the television show returned across the next few years. March 2000 saw the first time a companion originated in Big Finish itself, with the introduction of Evelyn Smythe, played by Maggie Stables; October 2000 saw Lisa Bowerman reprise her role of Bernice Summerfield from Big Finish's spin-off of the same name, a character who originated in the Virgin novels; and November 2000 saw the introduction of Frobisher, who originated in the comics, with Robert Jezek playing the role. Since then, every televised companion has reprised their role in Big Finish (with the exception of Jackie Lane's Dodo Chaplet).