Polypropylene (PP) film has been used in capacitors since 1970s. The high breakdown strength, low dielectric losses, and high availability make PP well suitable for use as capacitor dielectric. At the moment, the typical energy density achieved with PP film at room temperature is about $1.2J/cm^3$. Recent research on PP films focus on improving the permittivity, breakdown strength, thermal withstandability, and thermal conductivity, by producing copolymers, blending with other polymers, or adding inorganic particles. Some results claim energy density higher than double that of commercial bi-axially oriented PP capacitor film. Detailed research approaches and dielectric properties improvements are reviewed in this paper. The major focus is often placed on permittivity and breakdown strength. Unfortunately, little research shows positive results for maintaining the low dielectric losses.