The Tweenies consist of Bella, Milo, Fizz, Jake, Doodles, Izzles, Max, Judy, and are sometimes joined by Max's sister Polly. Other characters have appeared occasionally, including Milo's uncle, Mickey, his father, Jake's mother, Fizz's mother and father and Bella's grandmother. There have also been several animated characters.
The four main characters appeared in the first episode Tweenie Band, which was filmed in 1998 at Elstree Studios & aired on 6 September in 1999.
The Tweenies were played by only one set of actors, while the voices were provided by other actors. Unless otherwise indicated, all the following actors and voice talent were in the show from 1999 until its last episode, What Makes Summer? in July 2002.