"They Don't Know" is a song, written and composed by Kirsty MacColl, who recorded it in 1979. It became popular on radio in the United Kingdom, reaching #2 on the Music Week airplay chart. However, complications involving a strike at her distributors prevented the single from being shipped and promoted properly, preventing release of the single. MacColl's version of "They Don't Know" thus failed to appear on the official UK Singles Charts, which is entirely sales-based.
Although the song was not taken from an album, it later appeared on MacColl's compilation Galore (amongst others).
In 1983, Tracey Ullman, before she became better known in the United States as a comedy actress, included a version on her debut album, You Broke My Heart in 17 Places. This song was the album's most successful single, charting in the top ten of various national charts. MacColl performed backing vocals on Ullman's version and reprised her original "bay-ay-be-ee" in the middle of the song, which went to #2 in the U.K., #8 on the Billboard Hot 100 and #11 on the Adult Contemporary chart in the United States, and became the theme song for the final three seasons of Ullman's later television series Tracey Takes On....
It is sometimes erroneously reported that Ullman's version of "They Don't Know" simply uses the backing track from MacColl's original single. In fact, Ullman's version is an all brand-new recording, and the two versions are noticeably different: Ullman's version, which was produced by Peter Collins, is in a different key (C, as opposed to the original's B), and it features a very slightly faster tempo, a distinctly different arrangement and a guitar solo that differs substantially from the one played on MacColl's version, which was produced by Liam Sternberg. The confusion may stem from the fact that Ullman did use a previously-existing MacColl backing track when recording her own version of MacColl's "Terry" in 1984. (Both versions of "Terry" were co-produced by MacColl.)