Barcelona is a friendly city, there are few violent crimes. However, many tourists and even experienced travellers and residents get pickpocketed in Barcelona, therefore sometimes called the "capital of pickpockets". This is the biggest risk you will face. Crowded places and metro/busses and areas such as Raval and the famous Rambles are still the most likely places to get pickpocketed, but if you are just arriving be aware of pickpockets at the bus terminal Estació del Nord and train station Sants Estació. Take particularly care of your backpack and handbags. Also be very careful at internet cafes. At the airport, even in the arrival hall, you may be approached by individuals posing as foreigners and claiming they have been robbed during a train ride or similar, and asking you for some 50 or 100 Euros that are missing for their ticket home.
If you need to use an ATM, especially in very tourist heavy areas, use caution. When possible, use ATM in less crowded areas just off the main street. Scams have been known to happen involving ATM and PIN number theft. Be sure to stand directly in front of the machine and do not let yourself get distracted until your transaction is complete, and your cash and card safely stowed. Do not pay any mind to anyone trying to "help" you retreive a card that seems stuck in the machine by imploring that you enter your Pin number until it comes out. At this point your card is already stolen and you should proceed directly to the nearest phone to cancel said card.
Often a version of Three Card Monte is played on Las Ramblas. This game involves a dealer placing an object under one of three cups. The dealer then moves the three cups around mixing them up. The gambler tries to keep track of which cup the object is located under. After the cups have been mixed up, the gambler is given a chance to pick which cup the object is under, if the gambler chooses correctly he/she wins the amount gambled (Usually €50). However, it is not possible to win. This is a complete scam. Sure you will see people winning, and winning a lot of money. Those people are in on the scam with the dealer. Whatever you do, don't gamble, just watch and make sure not to make anyone involved mad. Remember there are more people involved than you realize.
Other areas of the city are less secure than average, such as Plaça Reial and the Raval and indeed the whole of the old town.
Women traveling alone should exercise caution while exploring the more isolated parts of Montjuïc. The city beaches, particularly the ones adjoining Barceloneta, have proven to be quite lucrative for bag-snatchers. Anything that one would rather not chance losing is best left (locked) in one's hostel or hotel.
Men traveling alone should expect the prostitutes on Las Ramblas in the early hours to be very aggressive, and are in with thieves and robbers. Stories abound of guys' belongings being relieved while their pants are down in dark back-streets.
Be aware of foreign matter such a bird poop that mysteriously shows up on your (and your companion's) clothing. Someone will soon offer to help you clean it off.
There have also been incidents of bag snatching while stopped at the traffic lights whereby the thieves open the car doors and take what they can. Please make sure that you always have your car doors locked during both the night and the day.
In case you want to report a crime - which you need to do to claim on travel insurance - be prepared for the reality that in the downtown police station, officers may not be able (or willing) to speak English, despite that fact the official theft report form is in both English and Spanish. The police station most often used to report theft is on Las Ramblas.