Transparency of polymers is a key property: very high purity is required for any glass-replacement and optical applications; while a slight haze is sometimes appreciated for special aesthetical effects. Additives and Formulation technologies significantly evolved. Polyolefin clarifiers are so efficient that transparent PP can now compete with naturally transparent polymers. Nanofillers are combining properties enhancement (gas barrier, mechanical strength) with high refractive index. However, despite these advances formulating plastics and elastomers also means bringing a thermal history to the resin, thus leading to partial degradation and loss of transparency. Finding the right formulation for transparent polymers is still a huge challenge.