Stalking can be defined as repeated and unwanted attention, contact, harassment or any other behavior directed at a specific individual that would cause a reasonable person to be fearful.  As private investigators, we are frequently contacted by clients who are dealing with a stalker situation.  Many, if not most, stalkers have mental issues.  They are often sociopaths or suffer from some other form of mental illness such as clinical depression, paranoia, substance abuse or personality disorders.  Below is a link to an article that provides some good background on this growing problem.  Remember, as Sun Tzu warned, "If you know yourself but not the enemy, for every victory gained you will also suffer a defeat."