A crucial aspect central is the relevance of multiple scales: phenomena occur at various length-and timescales, some of which decouple. This simplifies the tackling of complex systems: to build simpler models and focus solely on the relevant degrees of freedom. Scale separation takes its roots in renormalization group theory, and with significant implications in various aspects of theory (e.g., scaling concepts in polymer physics) as well as computer simulations (i.e., multiscale modelling). Figure illustrates the benefits of multiscale modelling for two applications: high-throughput screening of drug-membrane permeability, and a hierarchical description of polymeric organic electronics. Charges are transported primarily along the backbone of the chains, while the aliphatic side chains are needed to process the material. Panel reproduced from .