The most promising approach to effectively disperse organoclays into polyolens matrices at nanometric scale is the in situ polymerization. This approach can eliminate both entropic and enthalpic barriers associated with intercalating polar silicates with nonpolar polymers, which are drawbacks for achieving polyolen nanocomposites with good dispersion through melt blending. The in situ polymerization involves the intercalation of the catalyst within the silicates and the growth of the polymer chains between the intercalated clay layers (Figure 6.1).1 This chapter will concentrate on the effect of clay treatment on the in situ generation of polyolen nanocomposites with two families of modern homogeneous catalytic systems, early transition metal catalysts, namely, metallocenes (Figure 6.2a) and late-transition metal catalysts (Figure 6.2b).