Beware of pickpockets as they tend to operate in tourist areas, keep your belongings secure at all times and be careful if strangers try to make conversation or distract you as it could be an attempt to steal your valuables.
Credit card fraud, cloning cards can be an issue be careful where you use your cards. Also if you are withdrawing money from cash machines make sure you cover your pin and be discreet about the amount of cash you withdraw.
Some areas on the outskirts of Lisbon are becoming more dangerous, but it is unlikely the average tourist will visit these areas. The most common crime against tourists is pickpocketing and theft from rental cars or on public transport.