This article reviews observational and theoretical studies on seismic gap and seismic quiescence from the view-point of earthquake forecast. “Seismic gap of the third kind” by Y. Ishikawa is also briefly discussed. Seismic gap, which is recognized as an unruptured segment of plate boundary and large-scale geological structure in the present seismic cycle, is a strong tool for identifying the zone of high seismic potential in the near future. However, the concept of characteristic earthquake is not always valid. We need reliable information on the duration of seismic cycle, slip distribution of past earthquakes, and seismic coupling for effectively applying the seismic gap hypothesis to earthquake forecast. Seismic quiescence has been reported prior to major seismic events of a wide magnitude range, from great earthquakes to rock fractures in the laboratory, but the physical mechanism is not yet established, Among the several hypotheses so far proposed, the stress relaxation model based on the laboratory-driven friction law is of particular interest since it theoretically predicts the formation of seismic quiescence as a natural consequence of the physical process of earthquake preparation. For checking the validity of the model, we need detailed studies on the time-space development of quiescence, and focal mechanism change in the surrounding area. In spite of some limitations described above, past studies demonstrate that a combined use of the seismic gap and seismic quiescence is expected to provide a most useful tool to forecast the occurrence of a large earthquake for the time range of a year to several decades.