Physical fitness is a state of health and well-being and, more specifically, the ability to perform aspects of sports, occupations, and daily activities. Physical fitness is generally achieved through proper nutrition, moderate-vigorous physical exercise, and sufficient rest along with a formal recovery plan.

Before the Industrial Revolution, fitness was defined as the capacity to carry out the day's activities without undue fatigue or lethargy. However, with automation and changes in lifestyles, physical fitness is now considered a measure of the body's ability to function efficiently and effectively in work and leisure activities, to be healthy, to resist hypokinetic diseases, to improve immune system function, and to meet emergency situations.

Fitness is defined as the quality or state of being fit and healthy. Around 1950, perhaps consistent with the Industrial Revolution and the treatise of World War II, the term "fitness" increased in western vernacular by a factor of ten. The modern definition of fitness describes either a person or machine's ability to perform a specific function or a holistic definition of human adaptability to cope with various situations. This has led to an interrelation of human fitness and physical attractiveness that has mobilized global fitness and fitness equipment industries. Regarding specific function, fitness is attributed to persons who possess significant aerobic or anaerobic ability (i.e., endurance or strength). A well-rounded fitness program improves a person in all aspects of fitness compared to practicing only one, such as only cardio/respiratory or only weight training.

A comprehensive fitness program tailored to an individual typically focuses on one or more specific skills, and on age- or health-related needs such as bone health. Many sources also cite mental, social and emotional health as an important part of overall fitness. This is often presented in textbooks as a triangle made up of three points, which represent physical, emotional, and mental fitness. Physical fitness has been shown to have benefits in preventing ill health and assisting recovery from injury or illness. Along with the physical health benefits of fitness, it has also been shown to have a positive impact on mental health as well by assisting in treating anxiety and depression. Physical fitness can also prevent or treat many other chronic health conditions brought on by unhealthy lifestyle or aging as well and has been listed frequently as one of the most popular and advantageous self-care therapies. Working out can also help some people sleep better by building up sleeping pressure and possibly alleviate some mood disorders in certain individuals.

Developing research has demonstrated that many of exercise's benefits are mediated through the role of skeletal muscle as an endocrine organ. That is, contracting muscles release multiple substances known as myokines, which promote the growth of new tissue, tissue repair, and various anti-inflammatory functions, which in turn reduce the risk of developing various inflammatory diseases.

The 2018 Physical Activity Guidelines for Americans were released by the U.S. Department of Health and Human Services to provide science-based guidance for people ages 3 years and older to improve their health by participating in regular physical activity. These guidelines recommend that all adults should move more and sit less throughout the day to improve health-related quality of life including mental, emotional, and physical health. For substantial health benefits, adults should perform at least 150 to 300 minutes of moderate-intensity, or 75 to 150 minutes per week of vigorous-intensity aerobic physical activity, or an equivalent combination of both spread throughout the week. The recommendation for physical activity to occur in bouts of at least 10 minutes has been eliminated, as new research suggests that bouts of any length contribute to the health benefits linked to the accumulated volume of physical activity. Additional health benefits may be achieved by engaging in more than 300 minutes (5 hours) of moderate-intensity physical activity per week. Adults should also do muscle-strengthening activities that are of moderate or greater intensity and involve all major muscle groups on two or more days a week, as these activities provide additional health benefits.

Guidelines in the United Kingdom released in July 2011 include the following points: The intensity at which a person exercises is key, and light activity such as strolling and house work is unlikely to have much positive impact on the health of most people. For aerobic exercise to be beneficial, it must raise the heart rate and cause perspiration. A person should do a minimum of 150 minutes a week of moderate-intensity aerobic exercise. There are more health benefits gained if a person exercises beyond 150 minutes. Sedentary time (time spent not standing, such as when on a chair or in bed) is bad for a person's health, and no amount of exercise can negate the effects of sitting for too long. These guidelines are now much more in line with those used in the U.S., which also includes recommendations for muscle-building and bone-strengthening activities such as lifting weights and yoga.

Cardiorespiratory fitness can be measured using VO₂ max, a measure of the amount of oxygen the body can uptake and utilize. Aerobic exercise, which improves cardiorespiratory fitness and increase stamina, involves movement that increases the heart rate to improve the body's oxygen consumption. This form of exercise is an important part of all training regiments, whether for professional athletes or for the everyday person.