Which poet wrote 'Do not go gentle into that good night'?

Dylan Thomas, and it is a villanelle - a nineteen-line form with two refrains that return in a fixed pattern and both land in the final stanza. The form is old, French and mostly a curiosity, and this is the poem that made it famous in English.

The constraint is doing the work. A villanelle repeats itself by rule, and a poem about refusing to accept a death circles back to the same two lines whatever it says in between - so the form enacts the argument. Thomas wrote it for his dying father, a schoolteacher who had lost his sight and his fierceness, and did not show it to him.

Thomas died at 39 in New York, after a drinking bout, of what was recorded as pneumonia and misdiagnosis - he had been given morphine while in respiratory distress. The refrain has since been used in political speeches, on spacecraft, in films and at more funerals than any poet could reasonably want, which is a strange fate for a poem written to tell one man not to be calm about dying.