The genetic relationships among Formosan languages are discussed using a rule-history approach. The main results include: (1) the identification of shared innovations and interactions among Formosan languages; (2) the establishment of Formosan subgroups; (3) reconfirmation of the Tsou-Rukei relationship; (4) reordering of the Paiwanic and Atayalic languages; (5) rejection of the Formosan Hypotheses for the origin of the Chamorro language in Guam.