The US-based Electron-Ion Collider (EIC) is a future high-luminosity, polarized ep and eA collider dedicated to the physics of the nucleon/nucleus structure. I will give a theory overview of the science cases for the EIC. A special emphasis is placed on the partonic origin of the mass and spin of the nucleon, namely, how they can be understood in terms of quarks’ and gluons’ degrees of freedom. I will give a review of the mass and spin decompositions in QCD and discuss possible experimental observables