In the laws of war there are requirements that mercenaries must meet.
1- A mercenary shall not have the right to be a combatant or a prisoner or war.
2- A mercenary is any person who:
a) is especially recruited locally or abroad in order to fight in an armed conflict
b) does, in fact, take a direct part in all hostilities
c) is motivated to take part in the hostilities essentially by the desire of private gain and, in fact, is promised, by or on behalf of a party to the conflict, material compensation substantially in excess of that promised or paid to combatants of similar ranks and functions in the armed forces of that party
d) is neither a national of a party to the conflict nor a resident of a territory controlled by a party in this conflict
e) is not a member of the armed forces of either party to the conflict
f) has not been sent by a state which is not a party to the conflict as a member of the armed its armed forces on official duty
One of the most infamous mercenaries is Simon Mann who worked for Sandline during the Sierra Leone civil war. He tried overthrowing the government and was sentenced to 34 years in prison. He was let out in 2009 on Humanitarian grounds.
Private Military Contractors are extremely effective but very controversial in many opinions. These companies are paid and then when they get working they actually get everything under control relatively fast. I'm not sure as to how many "contracting companies" are left but I do know that there are still a couple big ones like DynCorp. DynCorp works the same way that Executive outcomes and Sandline did. Both of which closed their doors around the late 90's early 2000's.
These are Sandline Recruits in Sierra Leone