"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 recorded a version of the song. Already well known in the UK as an actress and comedian, "They Don't Know" was issued as her second single there, reaching #2. MacColl performed backing vocals on Ullman's version and reprised her original "bay-ay-be-ee" in the middle of the song, because Ullman had a very limited high-end range. The song was also included on Ullman's debut album, You Broke My Heart in 17 Places.

The track was released in the United States the following year, as Ullman's first single there. At the time, Ullman was almost a complete unknown in the US, as her TV appearances had not been seen in that country. Publicity for the single was aided by the fact that the video for "They Don't Know" featured a cameo from Paul McCartney; at the time it was filmed, Ullman was filming a minor role in McCartney's film Give My Regards to Broad Street. "They Don't Know" eventually reached #8 on the Billboard Hot 100 and #11 on the Adult Contemporary chart in the United States.

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, 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.)

...
Wikipedia