A personal trainer is a professional who educates people about physical fitness. Personal trainers typically design exercise routines and teach physical exercises to their clients. While some personal trainers work with only one client each session, others also teach groups of clients.

Personal trainers typically work with clients to improve body composition (weight loss or muscle gain). They may also be hired for more specific goals, such as an increase in strength, muscular endurance, cardiovascular endurance, or flexibility. Some trainers are qualified to improve sports performance, including speed and power. While not as common, some trainers may also be qualified to work with people who need help with physical dysfunction, including the improvement of balance, range of motion, knee and shoulder issues, and those released from physical therapy (please see "applicable populations" below and check the certification of your fitness professional).

Personal trainers work with clients on several time intervals. Some clients meet for a single session to answer questions and to develop an exercise program (or to ensure that their existing program is balanced). Others prefer to work with a trainer for several months for the purposes of motivation, variety, exercise design, or to work toward a specific goal. And still others work with a trainer indefinitely for motivation, accountability, variety, or to ensure consistent progress.