The atomic physics of excitation, ionization, and recombination is the story of frolicking electrons – like space traveling aliens, these little leptons are busy jumping up and down when bound to their “home planet” atom/ion. Launching freely into space, they adventure out and engage in a series of friendly energy exchanges with fellow particles and photons in an expansive plasma. Landing on and being captured by some other random “planet” atom/ion, the cavorting continues. In this chapter, we follow the dynamic lives of electrons, photons, and ions and present an abridged review of the physics of collisional excitation, ionization, and recombination. We describe photoionization, Auger ionization, direct collisional ionization, excitation auto-ionization, radiative and dielectronic recombination, and charge exchange. We show that detailed balancing and reaction cross sections, rates, and rate coefficients are the heart of chemical-ionization modeling of absorbers. We then present the cosmic photoionization rate of HI, HeII, MgII, CIV, and OVI as a function of redshift. We conclude with a comprehensive treatment of the heating and cooling functions of astrophysical gas.