In this presentation, we review the basic properties of coronal mass ejections (CMEs) and their impacts on space weather experienced at Earth. CMEs are transient outbursts of solar plasma and magnetic fields. Near the Sun, they have a scale size of several solar radii. As CMEs propagate outward through interplanetary space they expand and at the distance of Earth’s orbit, they can be as large as 0.25 of the ellipse encompassed by Earth’s orbit. The associated space weather effects threaten critical infrastructure on which society functions, including the integrity of power grids, satellite communication and navigation, and spacecraft operating in orbit around Earth.