For the criminal charges, there are several differences. One difference is that stalking requires more than one offensive contact, where harassment does not. Also, the penalties are likely going to be greater for stalking, which is a Class A misdemeanor (or Class C felony if one has been previously convicted), while harassment is normally a Class B misdemeanor unless certain other elements are met. There is also a mental state required to prove stalking as the person has to be engaging in the unwanted contact "knowingly."
A person commits the crime of stalking if:
(a) The person knowingly alarms or coerces another person or a member of that person’s immediate family or household by engaging in repeated and unwanted contact with the other person;
(b) It is objectively reasonable for a person in the victim’s situation to have been alarmed or coerced by the contact; and
(c) The repeated and unwanted contact causes the victim reasonable apprehension regarding the personal safety of the victim or a member of the victim’s immediate family or household.
A person commits the crime of harassment if the person intentionally:
(a) Harasses or annoys another person by:
(A) Subjecting such other person to offensive physical contact; or
(B) Publicly insulting such other person by abusive words or gestures in a manner intended and likely to provoke a violent response;
(b) Subjects another to alarm by conveying a false report, known by the conveyor to be false, concerning death or serious physical injury to a person, which report reasonably would be expected to cause alarm; or
(c) Subjects another to alarm by conveying a telephonic, electronic or written threat to inflict serious physical injury on that person or to commit a felony involving the person or property of that person or any member of that person’s family, which threat reasonably would be expected to cause alarm.
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Every state has its own statutes specifically defining these criminal acts. In general, however, stalking includes behavior that does not necessarily involve interacting with the victim (spying, following, checking credit histories, etc.) while harassment generally involves more personal acts of direct interference. Stalking is also usually more covert, while harassment is more often open and visible. These are broad concepts and there can be significant overlap, both definitionally and in the acts as, for example, in the case of threatening letters.
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