  Understanding the crossover from quantum to classical transport phenomena has
become of fundamental importance not only for technological applications due to
the creation of sub-10nm transistors - an important building block of our
modern life - but also for elucidating the role played by quantum mechanics in
the evolutionary fitness of biological complexes. This article provides a basic
introduction into the nature of charge and energy transport in the quantum and
classical regimes. It discusses the characteristic transport properties in both
limits, and demonstrates how they can be connected through the loss of quantum
mechanical coherence. The salient features of the crossover physics are
identified, and their importance in opening new transport regimes and in
understanding efficient and robust energy transport in biological complexes are
demonstrated.
