A study was made of the inclusions in 133 specimens of cassiterite and associated minerals from world wide Sn deposits. From the distribution of inclusion types, it would appear that most pegmatitic Sn deposits have been formed from siliceous melts, but that most other types of deposits have been formed from aqueous solutions. Since these aqueous solutions had the properties of liquids rather than gases, the deposits are more properly classified as "hydrothermal" than "pneumatolytic." Filling temperature measurements on fluid inclusions indicate that most hydrothermal Sn deposition occurs in the 300-500 degrees C. range. Of the previously suggested methods of Sn transportation, alkali-stannate and alkali thiostannate solutions are in accordance with the inclusion data. Additional possibilities are alkali of chloro- and alkali fluo-stannate solutions.