Theoretical models of stellar magnetic activity constructed to explain the features detected in complementary earth-based and space-based observations of the sun and stars are surveyed. The observational data are briefly characterized, and consideration is given to linear and nonlinear stellar dynamos, dynamo models of main-sequence stars, multimodal dynamos, and nonlinear global stellar dynamos. It is found that most models predict the observed increase in magnetic activity with the angular velocity of rotation and with the B-V color index. Although linear models are shown to reproduce most of the activity mechanisms, it is predicted that further development of nonlinear models will eventually give better results for the more complex phenomena. Outstanding observational and theoretical problems are listed and discussed.