"Wonderful Tonight" is a ballad written by Eric Clapton. It was included on Clapton's 1977 album Slowhand. Clapton wrote the song about Pattie Boyd. The female vocal harmonies on the song are provided by Marcella Detroit (then Marcy Levy) and Yvonne Elliman.

Answer this question based on the passage: who was you look wonderful tonight written for? Let's have some stream of consciousness first.
This is the relevant information: Clapton wrote the song about Pattie Boyd. So, the answer is Pattie Boyd.