There is little to no evidence that polygraph tests can catch liars, and quite a bit of evidence against it. Polygraphs work primarily by measuring skin conductance, which spikes upwards in the presence of sweat. But experienced liars do not sweat. Indeed, numerous double agents and spies in the US Government have evaded numerous polygraph tests effortlessly. Other variables measured by the polygraph, including changes in heartbeat and blood pressure, can also easily be faked.
The famous CIA spy for the Soviet Union, Aldrich Ames, passed polygraph tests on numerous occasions. He asked his Soviet handler what to do about these tests, and the handler only laughed, "Don't worry, they don't work." He was right.
Richard Nixon once famously quipped, "I don't know anything about lie detectors other than they scare the hell out of people." The law enforcement agents and police officers who use polygraph machines are not scientists - they are merely told the machine can be used to detect lies, given a few weeks of training, and set on their merry way, with little reason to question what they are told. Unfortunately, negative results on scientifically invalid lie detector tests can brand someone for life and ruin their career. False positives on polygraph tests are common.
In fact, polygraph tests are even harsher on honest people, who are not accustomed to having their word questioned. Experienced liars, trained to hide facial expressions and look calm at all times, have the upper hand when it comes to passing the test. Government agencies are given a false sense of security when polygraph tests become mandatory, leading to laziness in other areas of security. This is extremely unhelpful in areas where officers are given sensitive national security information and are trusted with it.
The polygraph test is based on a fundamentally questionable scientific assumption - that there is a universally consistent correlation between lying/truth-telling and physiological states. In-depth studies have demonstrated that there isn't.