**1. Introduction**

Normally, you already know the polyimide (PI) and the super engineering plastics derivative from PI are one of best high resistive dielectric materials for any application, for example print board, motor windings, surface materials of spacecraft, and so on. However, sometimes, the PI have breakdown phenomena under high voltage application with high temperature and humidity condition. It is considered that the origin of breakdown is the electric filed enhancement in the bulk or high growth of conduction current during the bulk by the homo charges injection form the electrode, or the hetero charges accumulation due to hydrolysis by surround high humidity environment. Furthermore, concerning the radioactive rays irradiated PI, the huge space charge polarization is produced. And then, sometimes, PI has a breakdown phenomenon due to irradiation.

To understand the dielectric characteristic for the serve condition, it is important that the space charge accumulation phenomena are observed. Therefore, in this chapter, we introduce the space charge accumulation in the bulks under the high electric filed, high temperature, high humidity and radiation environment.
