The original SDI was fraught with difficulties. The principle, of course, was space based satellites that knocked out the enemy warheads using either lasers or projectiles. Unfortunately, lasers could be deflected if the Soviets were forward thinking enough to paint the warheads silver. Railguns were another possibility, which accelerates something like a depleted uranium slug to incredible speeds through a coil of plasma. Unfortunately, handling such an incredibly high temperature gas gave material scientists major headaches.
As for lasers, and Death Rays, these again have major problems. The problem is, even the most powerful laser loses power over distance due to the natural convection currents in the air which needs to be compensated for in order to keep the laser focused. Even then, the power required is such that you have to keep the laser on the target for a particular length of time before it burns the hole required. I recall seeing a test with a death ray mounted on a plane, but the equipment required as just too damn heavy.
Son of Star Wars has its own problems. The patriot missile system was a good propaganda weapon but little else. As someone said once, its principle is like shooting a bullet out of the air with another bullet. And then there are decoy warheads which the ICBM could deploy to confuse the system. Personally, I still don't buy it as a useful defence against rogue states - launching a limited attack on the US would guarantee its own destruction.
