Friction is an essential part of human experience. We need traction to walk, stand, work, and drive. At the same time, we need energy to overcome the resistance to motion, hence, too much friction costs excess energy to perform work, introducing inefficiencies. In the 21^st century, we are facing the dual challenges of energy shortage and global warming from burning fossil fuels. Therefore, the ability to control friction has become a top priority in our world today. Yet our understanding of the fundamental nature of friction is still lacking.

Friction has always been a subject of curiosity. Intensive study of the origin of friction began in the 16^th century, after the pioneering work by Leonardo da Vinci. Yet progress in understanding the nature of friction has been slow, hampered by the lack of instrument to measure friction precisely. Ingenious experiments performed by Amontons, Coulomb, and others have yielded important insights to build the foundation of our understanding. Beginning in the late 1800s and early 1900s, the advent of steam engines, locomotives, followed by the automobiles airplanes, and space exploration demands a clear understanding of friction and the ability to control it for the machinery to last. Significant progress on how to apply and control friction in engineering friction was made through trial and error. At the beginning of the 21^st century, a new dimension of nanoscale friction came into the picture in conjunction with the arrival of nanotechnology. Our understanding of atomic and molecular friction has been expanding rapidly. However, integration of the new found knowledge of nanofriction into engineering practices has been elusive. Why? What is the scaling relationship between atomic friction and macro-friction? Is it possible to predict friction at the macro-level from nanoscale results? Why nanofriction values often do not agree with the macrofriction values given the same materials pair? Could it be there is a length scale dependent characteristic friction value?

In engineering practice, progress since the 1980s has been slow. Most of the effort has been focused on lubrication research such as elastohydrodynamic theories and solid lubricants. Friction mechanisms and failures have received relative little attention while nanofriction received much of the attention.

Today, energy efficiency and renewable energy generation demand our immediate attention while we seek reduction in carbon emission. The ability to control friction becomes an essential step in seeking sustainable technologies. Friction, after all, is an indicator of energy efficiency. If we can reduce the unnecessary parasitic energy losses and increase our current energy efficiency, it will give us time to develop alternative energy sources. This paper examines our current understanding of friction, filling some voids with experimental data, and attempts to integrate the various pieces to identify the gaps of our knowledge, hopefully to spark new avenues of investigations into this important area.