Astronomy is unique among the sciences because it does not allow us to study our objects of interest directly in situ. Instead, we must rely on information that reaches us primarily through electromagnetic radiation. Today, a comparison of observations with theoretical spectra is an excellent way to test and verify our physical understanding and models. Achieving this requires not only solving the radiative transfer problem but also accurately modeling the complex physics of the different types of stellar atmospheres. In this talk, I will present the general-purpose state-of-the-art stellar model atmosphere code PHOENIX. It models spherically symmetric atmospheres and includes hydrostatic stratification, gas chemistry, non-LTE, and an extensive treatment of atomic, molecular, and dust opacities. The code also includes specific physics for a wide range of stars and stellar-like objects. I will present several applications of PHOENIX, including the modeling of supernova light curves. Finally, I will discuss the recent development of self-consistent modeling of stellar chromospheres using the time-dependent module of PHOENIX. This approach enables us to model the temperature inversion within the atmosphere as well as observational features of stellar activity, including Ca II H & K emission, radio emission, and the Wilson-Bappu effect.