Biomechanics (Greek: βίος + μηχανική = βιομηχανικἠ, Greece: εμβιομηχανική because βιομηχανική = industrial) is the application of mechanical principles to living organisms. This includes bioengineering, the research and analysis of the mechanics of living organisms and the application of engineering principles to and from biological systems. This research and analysis can be carried forth on multiple levels, from the molecular, wherein biomaterials such as collagen and elastin are considered, all the way up to the tissue and organ level. Some simple applications of Newtonian mechanics can supply correct approximations on each level, but precise details demand the use of continuum mechanics.

The application of biomechanical principles to plants and plant organs has developed into the sister field of Plant biomechanics. The many strands of plant biomechanics are described in a text book on the subject by Karl Niklas Plant Biomechanics: An Engineering Approach to Plant Form and Function.

Applied mechanics, most notably thermodynamics and continuum mechanics, and mechanical engineering disciplines such as fluid mechanics and solid mechanics, play prominent roles in the study of biomechanics. By applying the laws and concepts of physics, biomechanical mechanisms and structures can be simulated and studied. Such concepts are found in the field of Sports Biomechanics where we apply the laws of mechanics and physics to human performance in order to gain a greater understanding of performance in athletic events through modeling, computer simulation, stimulation, gesticulation, mastication and measurement. Elements of Mechanical Engineering (e.g. strain gauges), Electrical Engineering (e.g. digital filtering), Physics/Dynamics (e.g. moments of inertia), Computer Science (e.g. numerical methods) and Clinical Neurophysiology (e.g. surface EMG) are common methods used for the analysis.

Relevant mathematical tools include linear algebra, differential equations, vector and tensor calculus, numerics and computational techniques such as the finite element method.

The study of biomaterials is of crucial importance to biomechanics. For example, the various tissues within the body's organs, such as skin, bone, and arteries each possess unique material properties. The passive mechanical response of a particular tissue can be attributed to characteristics of the various proteins, such as elastin and collagen, living cells, ground substances such as proteoglycans, and the orientations of fibers within the tissue. For example, if human skin were largely composed of a protein other than collagen, many of its mechanical properties, such as its elastic modulus, would be different.

It has been shown that applied loads and deformations can affect the properties of living tissue. There is much research in the field of growth and remodeling as a response to applied loads. For example, the effects of elevated blood pressure on the mechanics of the arterial wall, the behavior of cardiomyocytes within a heart with a cardiac infarct, and bone growth in response to exercise, and the acclimative growth of plants in response to wind movement, have been widely regarded as instances in which living tissue is remodelled as a direct consequence of applied loads.

Chemistry, molecular biology, and cell biology have much to offer in the way of explaining the active and passive properties of living tissues. For example, in muscle contractions, the binding of myosin to actin is based on a biochemical reaction involving calcium ions and ATP.