Is it illegal to use Shakespeare quotes in a song?

It depends on the specific quote and how it is used. If the quote is used in a way that is transformative, it may be considered fair use and therefore not illegal. However, if the quote is used in a way that is substantially similar to the original work, it may be considered copyright infringement and therefore illegal. Ultimately, it is best to consult with a legal professional to determine whether using a specific Shakespeare quote in a song is legal.