Introduction.- Flare Stars in a Quiescent State: General Characteristics of Flare Stars.- Photospheres.- Chromospheres and Transition Regions.- Coronae and Stellar Winds.- Heating Mechanisms for Stellar Atmospheres.- Flares: General Description of Stellar Flares.- Temporal Characteristics of Flares.- Flare Energy.- Dynamics and Radiation Mechanisms of Flares.- Models of Stellar Flares.- Long-Term Variations in Activity of Flare Stars: Activity Cycles.- Evolutionary Changes.- Conclusion: Activity and Subphotospheric Magnetic Fields.- Activity and Magnetism of Stellar Atmospheres.- References.