Extreme space weather can be defined as an event with variability in solar, ionospheric, thermospheric or magnetospheric parameters that cause significant degradation of certain technological systems or pose a serious risk to human health. Space weather effects/impacts can be divided into five broad groups: 1) ionospheric effects; 2) ionizing radiation/plasma effects on s/c electronics; 3) ionizing radiation effects on human health; 4) atmospheric uplifting and satellite drag; 5) geomagnetically induced currents (GICs). Extreme space weather can be traced to their solar and interplanetary origins, including solar flares, interplanetary coronal mass ejections and shock waves, and solar energetic particles. By fitting the tail of the cumulative distributions of space weather parameters, it is possible to estimate the probability of extremes. Historical examples show that before the space age, the most common examples of extreme space weather were extreme geomagnetic storms with intense GICs in ground-based infrastructure including power grids. Advances in technologies are expanding the area of extreme space weather. With the number of space objects increasing dramatically each year, satellite drag effects are becoming a concern. With thousands of aircraft maneuvering in response to a geomagnetic storm, it could theoretically be possible to achieve extreme space weather conditions for an event that would otherwise be considered moderate.