One of the most influential movies when it comes to all the action movies, buddy-cop movies, vigilante flicks and many of the anti-hero characters that followed. Not to forget all the Italian poliziotteschi(crime) movies that hoped to emulate this classic. Clint Eastwood makes the smooth transition from westerns to gritty urban thriller. There's also some pre-politically correct Hollywood examples of racism thrown in to make the character more realistic. The psychopathic Scorpio character is a genuinely creepy degenerate that you can't wait to get his comeuppance and the ending is a nightmare to many of the more ultra-liberal types out there. A young girl gets offed, a schoolbus full of kids gets threatened and Harry does much gun-fu.
There were 4 official sequels to this and a whole mountainous heap of rip-offs.