A review will be presented of our current theoretical understanding of the ITCZ, tropical cyclones, and the MJO. In order to obtain a more unified view of the midlatitude and tropical atmosphere, these phenomena will be interpreted in terms of potential vorticity dynamics. Included in the talk will be a discussion of various ways to define potential vorticity in a cloudy, precipitating atmosphere, and how to "invert" this potential vorticity to obtain the balanced wind and mass fields.