What exactly do those initials after your trainer's name mean, and how much weight do they carry? With the help of most any computer search engine, the answer to the first question is relatively easy. Answering the second brings you face to face with the wide variation in education and training required by national certifying programs. Here are some examples from among the hundreds of such organizations in the field:

Aerobics and Fitness Association of America (AFAA). Offers a self-study program ($429) with a mandatory three-day workshop. Student grades are based on a written as well as a practical exam. CPR certificate required from a group such as the Red Cross or the American Heart Association. Trainer certification is good for two years; 15 continuing education units (CEUs) required every two years for renewal.

American College of Sports Medicine (ACSM). Widely regarded as the most rigorous national trainer certification program ($270 nonmembers). For even the most basic certification, that of health fitness instructor, applicants must have a degree from an accredited college or university in a health-related field such as kinesiology or physiology. They must also pass a written on-site test, demonstrate physical competence and be CPR-certified. After four years, recertification ($60) requires 80 CEUs.

American Council on Exercise (ACE). Basic-level certification requires CPR certification but no academic degree and no demonstration of physical competence. Offers written exam ($200) and optional two-day prep course ($199). Certification is good for two years. Renewal requires 1.5 CEUs (equal to 15 hours in an educational environment).

Cooper Institute (CI). Requires CPR and hands-on demonstration of competence but no academic degree. Offers choice of a live course ($695) or home study videos ($725). Renewal after three years requires 30 CEUs.

International Fitness Professional Association (IFPA). Offers applicants a choice of an on-site two-day seminar and written exam ($379) or distance learning with an open-book test ($429). No CPR or demonstration of hands-on technique required. After two years, renewal requires eight CEUs within the first two years, then eight credits per year after that.

International Sports Sciences Association (ISSA). Offers at-home training ($595) using the Internet, a video and printed course materials. Students who don't attend a recommended two-day seminar and test can take a longer open-book test. CPR certificate required. Renewal of certification after two years requires 16 CEUs.

National Association for Fitness Certification (NAFC). Offers certification through a home study program ($359). Students choose their own proctors for written exam. For the practical exam, students mail in a video of themselves teaching a session. Requires CPR. After two to four years (depending on test score), renewal requires 18 to 28 CEUs.

National Strength and Conditioning Association (NSCA). The only trainer certification program to be nationally accredited by the National Commission for Certifying Agencies. Requires CPR but no academic degree or hands-on demonsration of competence for its personal trainer certification. Home study program ($305 for nonmembers) followed by written test administered on-site. Renewal requires six CEUs in three years.

National Strength Professionals Association (NSPA). No academic degree required. For beginner to intermediate level certification, offers choice of two-day on-site training with written and practical exam ($325) or home study with open-book test ($275). Both versions require CPR certificate. Renewal after two years requires 24 CEUs.