At the beginning of the twentieth century, engineers and technologists would have recognized the importance of adhesion in two main aspects: First, in the display of friction between surfaces — at the time a topic of growing importance to engineers; the second in crafts requiring the joining of materials — principally wood—to form engineering structures. While physical scientists would have admitted the adhesive properties of glues, gels, and certain pastes, they regarded them as materials of uncertain formulation, too impure to be amenable to precise experiment. Biological scientists were aware also of adhesive phenomena, but the science was supported by documentation rather than understanding. By the end of the century, adhesion and adhesives were playing a crucial and deliberate role in the formulation of materials, in the design and manufacture of engineering structures without weakening rivets or pins, and in the use of thin sections and intricate shapes. Miniaturization down to the micro- and now to the nano-level of mechanical, electrical, electronic, and optical devices relied heavily on the understanding and the technology of adhesion. For most of the century, physical scientists were aware that the states of matter, whether gas, liquid, or solid, were determined by the competition between thermal energy and int- molecular binding forces. Then the solid state had to be differentiated into crystals, amorphous glasses, metals, etc. , so the importance of the molecular attractions in determining stiffness and strength became clearer.