The earthquake nucleation process is inherently complex, due to an involvement of several deformation mechanisms with multiple spatial and time scales. Natural fault hosts a wide spectrum of slip rate from fast- to slow-slip. Before large earthquakes, the number of smaller magnitude events often increases, retrospectively named foreshocks. Foreshock can be interpreted to be a physical process implying unlocking of fault by fast-slip mode. Recent seismic and geodetic studies of foreshock sequences suggest that partial unlocking of fault took place episodically through interplay between fast- and slow-slip modes before some large earthquakes such as the 2011 Tohoku-Oki, the 2014 Iquique and the 2016 Kumamoto earthquakes. The partial unlocking causes stress loading onto the nearby critically loaded fault segments, resulting to triggering of subsequent dynamic and unstable slip. Alternatively, the partial unlocking of fault enhances the strength weakening of the earthquake nucleation area through slip invasion or fluid migration, that ultimately initiates the subsequent dynamic rupture. However, the manner of the unlocking is “episodic”, not “smooth acceleration” which has been typically observed as nucleation phase in laboratory experiment and numerical simulation model having simple fault zone structure. This episodic manner precludes a possibility of forecasting the subsequent large earthquake with a high degree of accuracy. The triggering of a subsequent large earthquake on nearby fault segments depends on the areal extent of the critically loaded seismic patches and how close these areas are to failure, even though the partial unlocking by both fast- and slow-slip processes is observed. An important research area is the development of methods for assessing the degree of criticality within fault segments adjacent to already ruptured portions. In addition, earthquake triggering probability by slow-slip transient shall be incorporated into operational earthquake forecasting scheme as future challenge, especially during the latter phase of the inter-seismic periods.