Fitness trainers and instructors lead, instruct, and motivate individuals or groups in exercise activities, including cardiovascular exercise (exercises for the heart and blood system), strength training, and stretching. They work with people of all ages and skill levels. Employers prefer to hire certified fitness trainers and instructors. Personal trainers are often required to be certified to begin working with clients or with members of a gym or other type of health club. Group fitness instructors may be allowed to begin without certification, but employers often encourage or require them to become certified.