Set forth in utilitarian philosopher John Stuart Mill's famous essay On Liberty, the harm principle states in general terms that one's actions are only moral and just so long as their consequences do not harm others; conversely, actions can only be punished when they harm others. Moreover, it states that power can only be exercised over a "member of a civilized community" for the sole purpose of preventing harm to others.
Harm Principle limitations: the "member of a civilized community" in question must be an adult, fully in control of his or her actions. More specifically, they must not be coerced into said action, the agent must be mentally competent to make their decision, and they must be sufficiently informed.
Problems that arise
What exactly qualifies as "harm"?
Is something like pornography innately harmful? If so, to whom?
If I jump off a bridge, could one argue I am harming others (family members, friends, etc.)?
What about hate speech? Censorship? Who do we let decide just what is harmful?
Is Mill's principle overly broad? Specifically, couldn't it be said that since we are all living in a society (some more than others), nearly all our actions could be said to harm others (reference: my previous suicide example)
Government prohibitions against murder, rape, theft, and speeding qualify as examples of this theory at work in practice. Despite the problems that arise when critiquing Mills' argument, the Harm Principle has gained wide acceptance within the philosophy community and a basis for shaping many public policy issues.
See also: Jeremy Bentham's (Mill's friend/mentor) Calculus of Felicity.