Louis Sullivan (1856 - 1924) revolutionized architecture by designing the first skyscraper and he became famous by proclaiming that "form follows function". When x-ray crystallographers visualized the structures of proteins for the first time, the structural biology field embraced the view that "function follows form" as the 3D-architecture of proteins could unveil various aspects of their function. Despite the original "1 gene - 1 protein structure - 1 function" relationship, nowadays a far more complicated picture emerges where the flexibility and dynamics of a protein can play a central role in a multitude of functions. The ultimate form(s) that a protein adopt when interacting with (a) partner molecule(s) are the most biologically relevant and in this context Sullivan's quote is still appropriate: the conformation that the protein adopts follows from the function of that protein. Despite the fact that many well-characterized proteins have a well-folded structure, there is a growing interest in the conformational flexibility within proteins. This flexibility is also a balanced phenomenon: excess of flexibility can be detrimental for protein behaviour, as well as the lack thereof. Notwithstanding its importance, studying intrinsically disordered protein regions or conformational rearrangements can be a very challenging. Therefore, flexibility can be perceived as a friend or a foe, depending on the context. This e-book showcases the impact of the study of protein flexibility on the structural biology field and presents protein flexibility in the context of disease as well as its benign aspects. As detailed knowledge of the structural aspects of polypeptides remains essential to comprehend protein function, one of the future challenges for structural biology also lies with large macromolecular protein complexes. Also there the dynamics and flexibility are essential for proper functioning and molecular movement, which is an important aspect of living matter. This challenge stimulated the development of advanced techniques to study protein flexibility and the use of those techniques to address fundamental biological and biomedical problems. Those innovations should help us to unravel the intimate link between protein function and flexibility and explore new horizons.