As pointed out in historical sources, the movement and evolution of comets in the night sky have always deeply impressed and fascinated people for millennia because of their bright halos and elongated tails. This is one of the reasons why a short chapter is dedicated to them in this book. The formation of the extended comet halo around the comet nucleus, the neutral gas and dust tail as well as the collimated plasma tail comes from the outgassing caused by the solar radiation and the influence of the magnetized solar wind. For comets in near proximity to the Sun, fragmentation of the comet nucleus or tail breakups can be observed more frequently. Scientists have learned much new about the chemical composition and structure of a comet’s nucleus from ESA’s Rosetta mission, which studied Comet Churyumov-Gerasimenko in detail for 2 years beginning in 2014.