Barcelona is a relatively safe city. That being said there is a problem with pick pocketing and taking bags, mostly in the most popular tourist areas. With any metropolitan area you need to be aware of your surroundings. Don't flash expensive items and don't set anything down. There are many two person scams where one will bump into you while another grabs your wallet. Be smart, try not to stand out as a tourist. Leave your passport at the hotel and bring along a photocopy. Carry just enough money to get you through the days. No reason to be scared just pay attention. It's good information to know before your trip.