Tyrannosaurus rex is one of the most well know dinosaurs of all. It was the largest carnivore ever to walk the earth. That title has now been challenged, but most people love Tyrannosaurus anyway. There have been many species of Tyrannosaurus named, but the only one that I am considering valid at the moment is T. rex.
I am considering Nanotyrannus lancencis, Stygivenator, and Dinotyrannus megagracilis juvenile T. rex specimens, due to conversations on the Dinosaur Mailing List and Carr (1999, see above). This means that all tyrannosaurids in North America, in the late Maastrichtian are T. rex. Nanotyrannus is defined as a separate genus mainly on characteristics that should be considered juvenile. The main arguement for validity stems from the highter tooth count of Nanotyrannus, but I am told that this too can be a juvenile trait. Carr (1999) proves that Nanaotyrannus is at least a juvenile something. Dinotyrannus is currently considered to be a juvenile Tyrannosaurus by the Los Angeles County Museum, where it is housed.
Recently, it has been discovered that there is much more variation between individuals than was previously thought or expected. This is possibly the reason for the confusion.
Holotype skull of T. rex at the Carnegie Museum of Natural History