Book excerpt: Complexity occurs in biological and synthetic systems alike. This general phenomenon has been addressed in recent publications by investigators in disciplines ranging from chemistry and biology to psychology and philosophy. Studies of complexity for molecular scientists have focussed on breaking symmetry, dissipative processes, and emergence. Investigators in the social and medical sciences have focused on neurophenomenology, cognitive approaches and self-consciousness. Complexity in both structure and function is inherent in many scientific disciplines of current significance and also in technologies of current importance that are rapidly evolving to address global societal needs. Several of these multifaceted scientific disciplines are addressed in this book including complexity from the general and philosophical perspective, magnetic phenomena, control of self assembly and function in large multicomponent clusters, application of theory to probe structure and mechanism in highly complex molecular species, and the design of multifunctional nanoscale molecules of value in decontamination and solar fuels research. Each chapter is both a review and addresses some ongoing challenges, thus each should provide a good preparation for further work in these highly active areas of research endeavour.