High energy long life rechargeable battery is considered as key enabling technology for deep de-carbonization. Energy storage in the electrochemical form is attractive because of its high efficiency and fast response time. Besides the technological importance, electrochemical devices also provide a unique platform for fundamental and applied materials research since ion movement is often accompanied by inherent complex phenomena related to phase changes, electronic structure changes and defect generation. In this talk, I will discuss key chemical, electrochemical and mechanical considerations in all solid state batteries. With recent advances in characterization tools and computational methods, we are able to explore ionic mobility, charge transfer and phase stability in electrode and electrolyte, as well as their interphases in operando, and map out the complex relations in novel solid state materials for next generation energy storage and conversion.