There are approximately 700 skeletal muscles in the body, and together they comprise about 40% of the body mass in a noncorpulent individual. When at work, muscles can use up to 1000 times more energy (as indexed by rate of ATP consumption) than when at rest, generating also copious amounts of heat as a by-product. This chapter will examine various biophysical aspects of muscle contraction but will begin with an overview of muscle structure.