Spectroscopic studies of the upper atmospheres of the giant planets using in frared wavelengths sensitive to the H3+ molecular ion sho w that this species plays a critical role in determining the physical condition s there. For Jupiter, we propose that the recently detected H3+ electrojet holds the key to the mechanism by which the equatorial plasma sheet is kept in (partial) co-rotation with the planet, and that this mechanis m also provides a previously unconsidered source of energy that helps explain w hy the jovian thermosphere is considerably hotter than expected. For Saturn, we show that the H3+ auroral emission is ca. 1% of t hat of Jupiter because of the lower ionospheric/thermospheric temperature and t he lower flux of ionizing particles precipitated there; it is probably unnecess ary to invoke additional chemistry in the auroral/polar regions. For Uranus, we report further evidence that its emissio! n intensity is controlled by the cycle of solar activity. And we propose that H 3+ emission may just be detectable using current technolo gy from some of the giant extra-solar planets that have been detected orbiting nearby stars, such as Tau Bootes.