Personal trainers are health/fitness professionals who, using an individualized approach, assess, motivate, educate and train clients regarding their health and fitness needs. They design safe and effective exercise programs, provide the guidance to help clients achieve their personal health/fitness goals and respond appropriately in emergency situations. Recognizing their own area of expertise, personal trainers refer clients to other health care professionals when appropriate.
The International Health, Racquet, and Sportsclub Association (IHRSA) have a position statement regarding the hiring of certified personal trainers. That position statements includes language to the effect that the personal trainer should hold at least one certification through an accredited organization.
There are currently five organizations that have been accredited for personal training certification programs:
Another organization that is a world-wide leader in exercise and health is the ACSM. So, while they are currently not accredited, I believe their certification is well respected.