Hydrogen is a promising alternative energy source for next generation automobile engines that meet the concern of energy shortage and global environmental pollution. Hydrogen detection is an important associated technology to be developed. The recently developed amorphous ferroelectric thin film capacitive gas sensors with a largely improved sensitivity to hydrogen show a great potential for this associated technology. This review presents an overall picture of amorphous ferroelectric thin film hydrogen gas sensors. It focuses on the correlation among processing, microstructural evolution and electrical properties of amorphous ferroelectric thin films. An attempt is made to detail the hydrogen sensitivity and transient response of various prototype capacitive devices with respect to the quality of the films and the hydrogen kinetic processes in the Pd/ferroelectric heterostructure. Recent advances on the hydrogen interface-blocking model for amorphous ferroelectric gas sensors are also described. (C) 2003 Kluwer Academic Publishers.