Playboy is best known for being one of the main media sources of pictures of unclothed women in the pre-Internet era. In the actual Internet era, it's the source of this guide detailing the very limited set of circumstances where "catcalling" might be OK.
Earlier this year, non-profit Stop Street Harassment commissioned a study [PDF] that found 65% of women had experienced some form of street harassment. 23% had been touched sexually and 20% had been followed. The organisation highlights a large number of other studies that point out the problem many women (and some men) face while walking down the street.
As Playboy points out, this isn't a difficult problem to solve. If you don't know someone, don't shout lewd comments at them. If you haven't established a sexual relationship with a person, don't touch them. Under virtually no circumstances should you follow someone home. It's simple: treat women (and everyone) with respect.
Flowchart: Should You Catcall Her? [Playboy]