Pickpockets

Barcelona is Europe's pickpocketing capital. Never keep your wallet, cash or important documents in trouser pockets or in bag pockets: a money belt is an easy and inexpensive way to prevent being robbed. As always, be alert in crowded places, such as public transport, train and bus stations, La Rambla and Raval. People may approach you asking for change, or to change money. Just ignore them. If you are asked to change money, then official looking police may approach you afterwards to 'check' your wallet for ID, etc. These are not police, so be at your most vigilant or you might find they have taken a few cards or cash upon returning your wallet. If you are in a crowd of spectators watching street entertainment, beware of anyone getting suspiciously close to you.
Pickpockets use the football trick as the local specialty. At certain tourist hotspots, there are people who will try to show you a 'magic trick'. This involves tying a piece of string around your finger. While you are distracted (and your arm is effectively disabled), an accomplice will pickpocket you. It is also possible that criminals will pose as tourists and ask directions to approach their victims. Keep your distance and be careful in tourist places.
The subway is a hotbed for pickpocketing activity, which can range from simple opportunistic thefts to coordinated attacks. Be especially wary on the subway platforms at Sants train station and Sagrada Família. A group of men will come out of seemingly nowhere while you attempt to enter a subway car and block your entrance and exit in a coordinated manner, effectively pinning you against the doors while they close. They will act as if the car is just crowded and they are trying to get on as well, but, in reality, they have already gone through your pockets.
Once they take stuff, they quickly return to the platform and walk off calmly while you are trapped in the departing subway as they make sure they exit just before the doors cannot be reopened. Violence in these situations is rare, and in most cases the goal of the thieves is to rob you undetected. Stay vigilant: do not leave anything in a back trouser pocket (except maybe a map of the city). Hold on to your bag or purse at all times. Do not leave anything unattended while you sit in a cafe or restaurant.
One guy acts like reading a newspaper and is about to go into the subway gate (he's scanning his target). While you insert your subway card to enter and before the gate opens, that guy immediately enters his subway card also, which causes the gate to jam and alarm sound. Immediately 3-4 other people appear and, while acting like trying to help, (pointing to the gate telling you 'the door is jam') will try to snatch your backpack or wallet while you are still surprised. Check and make sure no one tailgates on you, or, simply, just let the guy go first.
While you are at an outdoor table of a café, don't leave your smartphone on the table. Someone will try to steal it. For example, a guy babbling for change with an unreadable poster in his hands, getting closer and closer to the smartphone until he eventually picks it up, passes it to a second guy that will run away with it.

Scams

People in Barcelona are often very friendly and love to practice their English, so don't be unfriendly. That said, you should, of course, be suspicious if someone approaches you in a touristy area speaking your language and asking you for help. This should put your guard up immediately. Do not be tempted to sign their petition, give them directions, or help them with their problem. You don't know anything about where you are, since you're a tourist, so you won't be able to help them in any case.
Professional scam artists exhibiting a high degree of coordination are active in many areas of the city. Be careful in tourist areas. A variety of methods are employed, including the No Change trick. A common scam involves fake cops who will show up ask to see your passport, then take your belongings at the first opportunity. The story varies, but they are almost certainly not real ones. When it happens, the best strategy is to just walk away instead of starting any sort of conversations with them. Another trick is that one seemingly confused person will ask you for directions, diverting your attention and then suddenly fake police will appear asking for your ID. This is a co-ordinated move to divert the attention and steal whatever is possible. If such incident happens, just walk away, without listening to any of their conversation. Stay alert, especially in busy tourist area near the Sants station and Plaça d'Espanya.
Another popular scam happens in the metro. A group of scammers (often middle-aged women) will take advantage of the fuss while people are entering the metro and surround a tourist, frantically asking for directions. Most tourists won't know what to say while one of the scammers empties their pockets. They will try to confuse the tourist while the metro stays in the platform, and will get out just before the doors are closed. When you realize you've been scammed, the train will have already left and they will be safely outside with your belongings.
The bird excrement scam is also common. One or more accomplices will secretly spray or throw a smelly liquid on you. When you look up thinking a passing bird has pooped on you, they will run up to you and tell you that they saw a bird poop on you. They will offer to help you clean up, and while you are cleaning they will go through your pockets and any bags you have set down. It is wise to beware of anyone who is attempting to touch a complete stranger.
A version of Three Card Monte is one of many common scams played on Les Rambles. There are also people holding petitions to install a wheelchair lift in locations with a lot of stairs. Once your signature is obtained they will then aggressively ask for a donation. Sometimes there can be crowds of children demanding money with hardly anyone else in the area, making it difficult to get away.

ATMs

Barcelona offers ATMs in many locations. Many provide a wide range of services (withdrawals, transfers, mobile credit recharges, ticketing, etc.), and most accept ATM/debit/credit cards of various banks. Choose an ATM in a secure or highly-public space (e.g., in a bank lobby or airport terminal) to avoid machines modified by criminals to skim/video your card data or where you might be robbed after use. Ensure early in usage that the ATM supports a language you understand. For a full discussion of safe/effective charge/debit/ATM card usage and their cost trade-offs, see Money.

Areas of caution

Be very careful in the Barcelona Sants train station where thieves prey on new arrivals, even on the platforms.
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 lose is best left, locked, in one's hostel or hotel.
Men traveling alone should expect the prostitutes on Les Rambles, St. Antoni, and Raval in the early hours to be very aggressive and in league with pickpockets and robbers.
Also, people need to be careful when leaving the bars of the Olympic Port late as there are many pickpockets around.
Women should be wary of wearing exposed jewelery such as gold chains and necklaces. People walking down a street may be attacked from behind by a thief who may grab the necklace and try to rip it off the woman's neck before quickly running away, often down a convenient side street. Be especially careful of seedy looking men on bicycles as there have been many grab and snatch assaults in recent years.
In the event of such a robbery, people will need to find the local police station to report the incident, especially if a travel insurance claim is going to be made. Don't expect any police action beyond the report though as these types of events are par for the course and arrests, even when made almost never lead to prosecution due to a slow, antiquated, and overburdened legal system.
Parts of Barcelona are covered by closed circuit TV surveillance, but only the more popular spots.

Cars

Tourist drivers may attract special attention, such as Red light bag snatch or Flat tire scams

Reporting crimes

If you need to report a crime (for example, to claim on travel insurance), be prepared for the reality that in the downtown police station, officers generally do not speak English, despite that fact the official theft report form is in English, Spanish, and Catalan. The police station most often used to report theft is the one underneath Plaça Catalunya beside metro station, where they have some translators for common world languages.