In Europe, a black cat crossing one's path is generally considered good luck, however, in the USA and parts of Europe (e.g. Spain and Italy), which saw witch hunts, their association with witches caused them to be considered bad luck. The church associated black cats (and sometimes, other animals of the same colour, or even white cats) as being the familiars of witches.
In Romanian and Indian culture, especially in the historical region of Moldavia in Romania and everywhere in India, one of the strongest superstitions still feared by many people is that black cats crossing their path represents bad luck.
