This chapter introduces the various techniques to asses the function of CFTR. The numerous functional interactions of CFTR and cellular properties affected by CFTR will be described initially. This will be followed by sections explaining the importance of patch clamping and double electrode voltage clamp experiments in Xenopus oocytes for expression analysis of CFTR, and the Ussing chamber technique to analyze CFTR in polarized epithelia. It is concluded that examining CFTR function should occur at different levels, starting with the intact epithelium and ending with isolated CFTR proteins.