Arjen Lucassen's main project, named after the sci-fi character created for the concept that has spanned all of their albums. The music is a mix of metal and 70s prog, with an all-star cast of guest musicians (most notably vocalists) filling out the lineup on each release. Each album is a sci-fi concept album following a fictional creature named Ayreon. Also of note is the extraordinary production on each release, done by Arjen himself. The most beautiful bass tone I think I've ever heard.
He successfully appeals to the tastes of both light neo-prog and prog metal fans. Not many bands have done so.