"What You Don't Know" is a 1989 song by the American girl group Exposé. It was written and produced by the group's founder, Lewis Martineé, and appears on Exposé's second album, What You Don't Know. Lead vocals on "What You Don't Know" were performed by Gioia Bruno.

Taken as the first single from the album, the song peaked at #8 on the Billboard Hot 100 chart in July 1989, extending the group's streak of consecutive top ten U.S. pop hits to five. On the Billboard dance music charts, the song reached #2 on the Hot Dance Club Play chart and #8 on the Hot Dance Singles Sales charts. It was the first single by the group to be certified Gold by the U.S. RIAA. In the United Kingdom, the song eked out two weeks at #99 on the UK Singles Chart in September 1989.

The music video features the group rehearsing with a band on stage, then near the end of the video the women rush back to be with their boyfriends.

The song features a prominent horn section accompanying the singers, and there is a guitar solo between the second and third verses. Although the group continued to have success on the dance charts, this was one of the final Exposé pop hits featuring a beat, with which they had become associated from most of the singles off their previous album, Exposure. Many of the pop songs released by the group from this point on were mid-tempo or tunes.

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