Season 1:

Passionate kissing between characters and implied sex.

Two characters are briefly shown having sex, yet nothing explicit is shown

Prostitution plays a small role towards the last few episodes

A video is briefly shown of rape in a school basement, but we later find out it was consensual sex. Nothing is shown and the video is shown briefly.

Season 2:

Passionate kissing between characters

A man is briefly shown having sex with a woman in the front seat of a car, both are clothed and nothing explicit is shown.

Season 3:

The third season is centered around a serial killer who targets young female prostitutes.

Some instances of semi-explicit sexual banter among adults.

There is discussion of child pornography tapes. The beginning of some of the tapes are shown briefly, but no disturbing images. The girls on the tapes are asked if they are virgins.

Two rape scenes -- one involving a young girl and a pimp; the other a young male and his probation officer. Nothing explicit is shown, but heavily implied.

Some kissing.

Season 4:

Implied suggestions of incest.

Nude shower hazing scene - A teenage boy's rear is shown as other boys force him to masturbate to a picture of his mother.

Sexual dialogues from teenagers.

Pictures are seen showing a teenage girl posing seductively.

Violent crimes and crime scenes are shown.

Dead bodies and crime scene photos are shown

The crime scenes can be very bloody at times, but never very gory.

A few mild fights during chase scenes

The show is overall not very graphic, but the content can be very dark and disturbing.

Some characters are shown at point blank range.

Some mild language -- more language in the second season.

Season 4: Strong and sometimes persuasive use of "fuck" and stronger language throughout

One character is shown using heroin.

Smoking by numerous characters.

A character is shown drunk.

Implied use of drugs or drug buying behaviour.

The show is incredibly dark and disturbing. The episodes are not very graphic, but intense, involving scenes of family grief over the murder of a family member and crime scene situations.