Spin transistors are a new concept device that unites an ordinary transistor with the useful functions of a spin (magnetoresistive) device. They are expected to be a building block for novel integrated circuits employing spin degrees of freedom. The interesting features of spin transistors are nonvolatile information storage and reconfigurable output characteristics: these are very useful and suitable functionalities for various new integrated circuit architectures that are inaccessible to ordinary transistor circuits. This article reviews the current status and outlook of spin transistors from the viewpoint of integrated circuit applications. The device structure, operating principle, performance, and features of various spin transistors are discussed. The fundamental and key phenomena/technologies for spin injection, transport, and manipulation in semiconductors and the integrated circuit applications of spin transistors to nonvolatile logic and reconfigurable logic are also described.