Watch Dogs

There's a total of 20 Criminals to takedown or kill. They'll show up while you're driving through the city or by hacking potential targets. They're are shown on the map by a blue triangle icon inside a blue circle.

You'll learn about profiling people by using your Smartphone very early on in the game. While you're doing so, you need to look out for and hack people that have the words 'Potential Crime' listed next to them. Once the hack is completed, the blue triangle icon inside a blue circle will appear. Head to this location, and somewhere within the circle a crime is about to take place.

Once you arrive within the area, bring out your Smartphone again and keep profiling people help you identify the potential criminal or victim but make sure you do not get caught in the process. Once you've profiled the criminal (not the victim), the ctOS on your Smartphone will give you the signal to intervene and take them down. You can choose to do this in a lethal manner or by simply knocking them out which will grant you a better reward. Now repeat 19 more times to unlock the achievement.

You can view how many you have left if you bring up your Smartphone, select the 'Statistics' App and then press to access the 'Progression' menu.