Leiden is a safe city that takes pride in the fact that it is clean. Many travelers visit Amsterdam before coming to Leiden, and they often remark on how different the two cities are. It's not necessarily that Amsterdam is dirty or unsafe, but there are a lot of sketchy individuals who live in the city because of the nightlife. Because of that attraction there are often criminals and pick-pockets all throughout the city. In Leiden, however, this is not the case. It is a much smaller city by comparison, and much of the negative connotations that people associate with Amsterdam do not carry over. You should still watch out for yourself, but only out of general precaution. You should never leave your wallet out on a table unattended. You definitely don't want to leave anything important behind in the room. You should always have your passport on you no matter what. Also, don't leave your camera slung around your neck, as this is a huge sign that you are a tourist. If you get into any trouble contact the Leiden police department, or if you are in real jeopardy contact the United States Embassy.