Geomagnetic indices are a measure of geomagnetic activity, which is a signature of the response of the Earth magnetosphere and ionosphere to solar forcing. They play a significant role in describing the magnetic configuration of the Earth’s ionized environment. In the second half of the twentieth century, they have become a key parameter in Solar Terrestrial studies; in the past 15 years, they have become a key parameter in Space Weather, being commonly used to detect and describe Space Weather events. The objective of this chapter is to contribute to a better understanding of the meaning, usefulness, potential and limitations of geomagnetic indices. Standard geomagnetic indices, as well as some newly introduced quantities are considered. We present for each index, or each index family, a short but complete description of the derivation process and a review of the information that the index may provide on the dynamics of, and on the physical processes that take place in the Earth’s ionized environment.