How accurate are lie detectors, and is the technology actually improving?

I’ve noticed that shows like The Jeremy Kyle Show (the UK’s version of Jerry Springer) put a lot of weight on lie detector results, sometimes claiming they’re “99% accurate.” Is there any truth to that, or is it just TV drama? Can lie detectors ever be perfected to the point where they’d be reliable in courts and criminal investigations? I know they’re a staple in movies and spy shows, but in reality I’m unclear on how these systems actually work, how effective they really are, and what’s being done to make them better.