The recent advances in the studies of the toughening methods and theories of polypropylene (PP)-elastomer blends are reviewed in the present article. Inclusions are key to toughening PP; they can play the role of agent-induced crazing, cause shear yielding of the matrix around them, and end the propagation of cracks. The major theories interpreting the toughening mechanisms of the blends are: multiple crazing, damage competition theory, shear-yielding theory, microvoids, and cavitation theories. The factors affecting the toughening effect are relatively complicated. Therefore, these theories have been verified only in some cases when they have been applied in relevant conditions. To achieve the objective of better toughening, it is important to improve the uniform distribution of dispersed-phase particle size and suitable filler size, as well as improving the dispersion of the inclusions formed in the matrix; in addition the matrix materials or fillers must be functional with suitable modifier in order to enhance the interfacial adhesion or to improve the interfacial morphological structure between the filler and matrix. However, the exact toughening mechanisms for PP-rubber blends have to be studied further because of complications resulting from the crystallinity of the matrix.