The show is filmed in Chicago. The actors/actresses trained for 3 months beforehand with retired S.W.A.T. personnel and real Chicago Police officers.
Shares a universe with other shows created by Dick Wolf: Law & Order: Special Victims Unit (1999), Chicago Fire (2012), Chicago Med (2015) and Chicago Justice (2017). This means that sometimes characters of Chicago P.D. appear on his other shows and its story continues there or vice versa.
A 1505 fund is an account devoted to money police had seized from people during drug-related investigation, money the department ultimately kept via the practice of civil forfeiture.
Each episode title throughout the series is taken from a line of dialog spoken in the respective episode.
In The Three Gs (2015) they bust a gang leader played by Brian Tee who eventually became Dr. Ethan Choi on Chicago Med (2015).