Contrails are anthropogenically induced clouds that have the potential to impact climate and reveal aircraft positions. Contrail formation is governed by temperature and humidity fields and is generally limited to temperatures below -40 C. Contrail growth and transformation into cirrus clouds is more complex and must be understood to fully account for contrail effects on climate. Contrail microphysical and optical properties as well as coverage, location, and illumination conditions are also critical for determining contrail radiative forcing, a climate change metric. In situ measurements, satellite remote sensing, cloud process models, and climate models are used to study contrails and their potential impacts on global climate.