On Earth, chlorine and bromine are distributed quite systematically in the large geological reservoirs. Due to their chemical character, both elements are primarily concentrated in the fluid phase of any system and as a consequence, they are highly concentrated in the oceans on earth. The concentrations in rocks and sediments are relatively low, except for evaporites where a considerable part of the chloride and to a lesser extend the bromide that was once stored in the oceans is concentrated (Schilling et al. 1978; Knauth 1998; Hay et al. 2006). The main geological reservoirs that can be recognised concerning the distribution of chlorine and bromine are, as already discussed in Sect. 3.2.1 , the oceans, the mantle, evaporites, sediment (pore water), the continental crust and the oceanic crust. Chlorine and bromine isotope variations in the hydrosphere and evaporites will be discussed in this chapter, the isotope variations in the other reservoirs will be discussed in the following chapters. The atmosphere is only a small reservoir. Still, the chlorine and bromine isotope measurements of chlorine and bromine present in the atmosphere are described in this book, as atmospheric chlorine and bromine have considerable influences on environmental processes. In the following chapters an overview will be given on our knowledge of chlorine and bromine isotope compositions of the terrestrial (and extraterrestrial) reservoirs, as well as their variations within these reservoirs. Chlorine isotopes have been studied, at least globally, in most of the reservoirs. Knowledge on bromine isotopes is still rudimentary, but sufficient information is available to present basic information on its distribution on earth.