Magnetic fields of planets and stars are believed to be generated and maintained by dynamo action, because the timescale of their dissipation is shorter than their lifetime. For dynamo processes to act, however, a seed field is required, which will be brought in from outside when the objects are formed or generated inside them. In this chapter, as preliminary studies of dynamo processes, Biermann’s battery mechanism and Cowling’s anti-dynamo theorem are introduced. Then, the essences of the mean field dynamo by Streenbeck-Krause-Rädler and Parker’s cyclonic dynamo are described.