It seems pretty simple: The Foreign Intelligence Surveillance Act is the law that U.S. intelligence experts use to sniff out foreign terrorists. Considering that it became law in 1978 -- and that technology has evolved quite a bit in the ensuing three decades -- modernizing this essential security tool to fit today's communications networks should be relatively simple.

It's not. Opponents of modernization have dragged out debate on what should be a no-brainer with wild-eyed "Big Brother" scenarios. They have poisoned the debate by using misleading terms such as "warrantless wiretapping" and raising a stink over proposals to give immunity to communications providers that cooperated with the government's electronic surveillance program.

Under FISA and other laws, domestic wiretapping -- that is, listening in on phone calls -- requires a warrant or order from a judge. Modernization doesn't change that.

Modernization is really about electronic surveillance -- looking at the e-mails, instant messages and other Internet activities of terrorists outside of the U.S.

The problem is that, unlike with wiretapping, there's no way to capture all of the Internet traffic of just one individual or group. A terrorist cell might keep e-mails on one server, run a Web message board on another and store bomb-making manuals on a third. These could be in different countries, and they could be accessed from anywhere in the world.

Because of the way the Internet works, many of these communications, even if they begin and end outside the country, still pass through the U.S. And all that traffic passes through the same pipe as everyone else's Internet communications. So the only way to get at foreign terrorists' communications is to tap the entire stream of traffic.

For modernization opponents, that's where the story ends. But for our intelligence agencies, that's just where it begins.

The often-ignored next step -- and the crucial one for Americans' privacy and the effectiveness of intelligence operations -- is "minimization." This is the process of filtering the stream of traffic to remove domestic communications and communications that don't have any intelligence value.

Minimization is imperfect -- on the Internet, it's hard to tell where a communication originates or winds up -- but it's the only way to get the job done, given the quantity of Internet traffic. And it protects privacy: Sophisticated algorithms can do a good job of automatically filtering out domestic communications.

The main issue in modernization is whether FISA, created to oversee domestic surveillance, should apply to this kind of surveillance of foreign communications, many of which just happen to pass through the U.S.

In a decision last summer, the court that administers FISA ruled that the law does apply. Congress subsequently passed a law overturning that decision. But then that law expired in February, throwing intelligence operations into uncertainty.

If FISA applies, our intelligence experts will have to obtain approval for foreign electronic surveillance requests to the FISA court, a cumbersome process that can take hundreds of hours per application. Under modernization proposals, intelligence agencies would just have to submit their minimization procedures to the court to make sure that they comply with the law and are tailored to minimize away domestic communications.

Acknowledging that modernization isn't really about wiretapping makes clear why immunity is so important. There's no way for our intelligence experts to capture foreign terrorists' Internet communications without the cooperation and the expertise of network providers, which are now facing nearly 40 lawsuits for working with the government. Legal liability will chill future cooperation.

Worse, these companies have no good way to prove their innocence, because doing so would require the disclosure of specific details about how the surveillance system works -- details that the government rightly claims are state secrets and won't allow the communications companies to make public.

If this debate were really about domestic wiretapping, modernization opponents might have a point about immunity. But it isn't. It's just about scoring political points.

Opponents should look at the facts and acknowledge that modernization is really about foreign electronic communications, not domestic wiretapping. That would be a quick way to break the stalemate in Congress and restore the intelligence authorities the U.S. needs to identify and monitor foreign terrorists.

Andrew M. Grossman is Senior Legal Policy Analyst in the Center for Legal and Judicial Studies at The Heritage Foundation.