THE construction of several large ground-based telescopes(1,2) and the anticipated launches of new space-based ones(3-5), have prompted renewed interest in the means by which extra-solar planets might be discovered(1,6,7-11). The direct detection of light from such a planet would be the most compelling means of discovery, and it may soon be technically feasible(1,6). Jupiter has traditionally been used as a benchmark for observability, but extrasolar giant planets could have a wide range of masses and ages(12), and could be significantly brighter than Jupiter. Here we present calculations estimating the optical and infrared fluxes of extra-solar giant planets with a range of ages, and demonstrate the conditions under which they will be observable with several new telescopes. Giant planets with masses greater than that of Jupiter, and younger than about 1 billion years, are the best targets, and they should be visible using the generation of telescopes now under construction.