Lasers can interact electrons with electrons in several ways. With a \ang{90} crossing angle the laser can be used as an unbreakable wire that will generate Compton scattering and allow the measurement of electron beams that are only a few micrometers wide. Compton scattering is also used when the beams collide head on to generate intense X-rays or $\gamma$-rays as was done in the MightyLaser experiment. Building a dedicated accelerator for this purpose will make a compact X-ray source, as in the ThomX project. Finally if the photons are co-propagating with the electrons in a plasma they create a wakefield that will accelerate the electrons with very high gradients, creating the need for new diagnostics
