NGOs and PMCs
Since the US invasion of Iraq, PMCs have become quite popular in providing security services for diplomatic missions, reconstruction efforts, private businesses, and some NGO activities. Companies such as Blackwater, Triple Canopy and Aegis (among others) have received lucrative contracts in providing private security services. There has also been a considerable amount of controversy over PMCs, with allegations of fraud and documented incidents where contractors have acted criminally against civilians with little accountability.
PMCs tend to employ contractors with past military or law enforcement experience. Contractors dress in civilian clothing (very similar to what a typical aid worker may be wearing, with the exception of body armor and prominently displayed weapons). Some PMCs and their staff have excellent reputations and do a very good job under difficult circumstances. Others have poor reputations and their presence may actually increase versus decrease risk.
The decision to use a PMC is going to be made at your organization's higher levels, so as a field security officer, you may not have much say in initial discussions. However you do have a say in whether you think the actions of PMC contractors may be compromising the safety and security of staff and programs. As a security officer you'll likely end up acting as a liaison with assigned PMC management and personnel. Take the time to understand their roles and responsibilities and spend time educating them about your programs and needs. The goal is to try to find a balance between sufficient security and effective programming.
Remember that a PMC is like any vendor and is simply supplying a service to your NGO. If the situation warrants, it may be necessary to suggest using another vendor. If that's the case, be sure to extensively document the reasons why before discussing with your organization's management.