This paper presents a test system for conducting online (pseudo–dynamic) tests on a real time–scale. A historical perspective is provided regarding the development of such testing, and a real–time online test system devised by the writer is introduced. The proposed system is characterized by (1) use of a digital signal processor now readily available; (2) adoption of the C language to ensure easy programming; and (3) separation of response analysis and displacement signal generation to allow the testing with complex structures. A five–storey base–isolated building model, treated as a six–degrees–of–freedom structure, was tested for various ground motions. The tests demonstrated that the system is able to simulate earthquake responses involving large displacements and velocities as well as complex numerical hystereses. Advantages and drawbacks of the real–time online test are discussed in reference to other experimental techniques: the quasi–static loading test using a predetermined loading history, the shaking–table test and the conventional quasi–static loading online test. Areas that require further research for advancement and refinement of the real–time online test are also noted.