This paper reviews the structure (from molecular to macroscopic level) of collagen, elastin and glycosaminoglycans, with special reference to their functional properties and to how their behavior can be elucidated from in vitro manipulations of the tissues by biochemical means. Modes to analyse the mechanical behavior are briefly discussed and exemplified by data from reconstituted collagenous "tissue". It is concluded that the basic equation for the "non-linear elasticity" component in Fung's law (and for the reconstituted "tissue" a modification of it) is a powerful tool for analysis of the physiological range of the stress-strain curve. Further, enzymatic "dissection" of one tissue element at a time provides a valuable method for the analysis of tissue element interactions. This approach is illustrated with data from aortic tissue. It is shown that the mechanical properties of the aorta depend on the interaction between elasticity and collagenous elements and that the strength of the tissue is not derived from its collagen fibers per se.