This book presents selected contributions from ICMFM XX and the Polish National Conference—KKMP. The XX International Colloquium on Mechanical Fatigue of Metals (ICMFM XX) was organized on 15–17 September 2021, in the Faculty of Mechanical Engineering of the Wroclaw University of Science and Technology, in Wroclaw City, Poland, in a remote form. Its aim was to facilitate and encourage the exchange of knowledge and experiences among the different communities involved in both basic and applied research in the field of fatigue of metals, looking at the problem of fatigue from a multiscale perspective, and exploring analytical and numerical simulative approaches, without losing the perspectives of the application. The Polish National Conference—KKMP 2021—was organized remotely with 50–80 prominent international participants from the fracture mechanics community.