“…We should finally point out again that we did not aim at writing a comprehensive review of the subject, therefore we left out many topics that we felt were too advanced for an introduction, such as the thermodynamics and resource theory aspects of complexity [43,46,151,152], the relation with bulk dynamics (in the sense of reconstructing the Einstein equations in the bulk from the complexity of the boundary) [85], alternative conjectures, most notably the one in [92] (sometimes called CV 2.0), complexity in de Sitter space [52,[153][154][155], the evolution of complexity after a quench [77,156], the relation between complexity and chaos [32,157], other notions of complexity such as the operator complexity [158,159]. We hope that our readers will be encouraged to delve further into this fascinating subject and contribute to its development.…”