Though current commodity superscalar RISC processors are very successful in axploiting functional parallelism within a chip and are able to support the built of small-scale multiprocessors, they fail in supporting large-scale parallel systems. Multithreaded architectures represent a promising new direction in processor design for supporting construction of scalable, massively parallel computers. In the current chapter a detailed study is given on the main classes and features of multithreaded architectures. Several prototype machines are also described in order to illustrate the main principles.