Abstract: The primitive equations (PE) are widely used in the study of geophysics, in particular when the aspect ratio of the domain is small, such as the ocean and atmosphere in the planetary scale. They are derived from the Navier-Stokes equations or Euler equations by taking the hydrostatic limit. The viscosity plays an important role in the properties of the PE. I will first discuss the key ideas in obtaining the 3D global well-posedness of the viscous PE, and then review several results for the inviscid PE, including the ill-posedness, the local well-posedness under the local Rayleigh condition, the finite-time blowup, and the effect of fast rotation. Finally, I will discuss some recent works on the investigation of fractional dissipative PE, which can be interpreted as the "bridge" between viscous and inviscid cases.