Natural rubber blends are continuously developed and grown in polymer science. These products have been widely used in industrial scale. Interpenetrating polymer networks (IPNs) are also one type of polymer blends that compose of two or more polymers with at least one is polymerized/crosslinked with crosslinking agent in their networks in both without and/or with the covalent bond between the chains same or of different polymer types. These systems can be designed and created to the new materials with improved the combined properties of their components. The classifications of these systems can be divided into two categories, i.e. miscibility and immiscibility. Miscible blends present as a single-phase of new material structure which observed one glass transition temperature. Immiscible blends present as a phase separation of new material structure which observed more than one glass transition temperature regions depending on the number and the concentration of their components. In some cases, however, there are in the intermediate stage called ‘compatibility’ which the immiscible systems are improved by the special processes for more miscibility, such as the addition of the compatibilizers, the grafting some parts of the polymer chains. Anyway, the miscibility of blends is difficult to identify by simple observation. For example, some blends are looked to be miscible blends but they are really immiscible. This miscibility can be predicted by the basic thermodynamic Gibbs free energy. Furthermore, the miscibility of these systems can also be analysed and confirmed by several techniques, such as thermal analysis (i.e. differential scanning calorimetry, dynamic mechanical thermal analysis, thermogravimetric analysis, etc.), microscopic analysis (i.e. transmission electron microscopy, scanning electron microscopy, atomic force microscopy), scattering analysis, and infrared spectroscopy. The miscibility of natural rubber blends and IPNs, and their properties were discussed in this chapter.