Crystallization is an important stage in the synthesis of materials and in the formation of minerals in natural environment. This review represented some typical non-classical nucleation pathways proposed in recent years, i.e., spinodal decomposition, pre-nucleation clusters, and amorphous precursors. The multiple nucleation pathways coexisting in the same system were discussed. The theoretical system construction of incorporating non-classical nucleation into classical nucleation theoretical framework was analyzed. In addition, the future research aspects of crystal nucleation were also prospected. This review provides a perspective aspect on crystal nucleation - classical and non-classical nucleation coexisting in a common system, which is promising to further clarify the theory of crystal nucleation.