The defect chemistry of protons in oxides is well understood, but rela
tionships between proton concentration and structure/composition are s
till rather empirical. The mechanism of proton transport has been clar
ified by various techniques, including simulations which now largely c
onfer with experimental data. New proton conductors are being develope
d mainly among complex perovskite-related oxides with disordered or cr
ystallographic oxygen deficiencies.