Like most big US cities, Philadelphia has its share of homelessness and crime, but prudent travellers are not at any undue risk. Certain neighbourhoods are seedier than others and are considered 'unsafe'. These include Germantown, parts of West Philadelphia and South Philadelphia, and most of North Philadelphia. Though the majority of the Center City and other touristed sections of the city are reasonably safe, travellers should be aware of their surroundings whenever they walk in the city. After dark, some of the city's parks - particularly Fairmount - host numerous crimes, some of them violent. Most tourists won't visit or even see Philly's most dangerous spots.
Standard urban common sense applies: leave valuables at home; avoid walking alone through poorly lit areas at night; and if you're lost and confused, try not to look lost and confused. If you find yourself somewhere you would rather not be, act confident and sure of yourself; then go into a store and call a taxi.