Fifteen Years of High Angular Resolution Studies of Mira's Atmosphere

I describe here results from high-angular resolution imaging studies of o Ceti (Mira). In 1983, we discovered that the atmosphere of the prototype of Mira-type variables is not symmetric. Since then, a number of multiwavelength high-angular resolution observations have confirmed the presence of asymmetries in Mira's atmosphere, and detected asymmetries in the atmospheres of other Mira-type variables. The high-angular resolution images of Mira obtained over the past fifteen years, including recent HST observations, show that the strength and shape of the asymmetries change as a function of wavelength and time. Plausible mechanisms for these asymmetries include hot spots, nonspherical pulsations, interaction with the companion and bipolar outflow. The presence of asymmetries in Miras could have serious impact on evolutionary models, and on the development of model atmospheres.