According to statistics Istanbul is one of the safest large metropoles in the world (together with Berlin). There is some pickpocketing in crowded places, but I never experienced it myself. Street violence is very rare and often relates to events like football matches or demonstrations. Police is also present in all crowded areas. Stay away from any political demonstrations at Taksim Square and avoid the murkiest side streets of Istiklal Street. Don’t leave your drinks/jackets unattended at crowded bars/restaurants. Never enter Tarlabasi area which was not the most safest part. Especially alone and in the nighttime.