Micelles are formed when block copolymers are dissolved in solvents selective for one of the blocks. In contrast to micelles formed by surfactants, block copolymer micelles are generally more robust, and this opens the door to many applications. This article examines the formation and structure of fiber-like or filamentous micelles, with cross-sections of nanometer dimensions. These fascinating objects are currently under investigation for drug delivery applications, as impact modifiers for plastics, as templates for the deposition of metal nanoparticles and as precursors to nanoscale ceramics. Moreover, in some cases, studies of their formation and fragmentation are beginning to provide insight into the generation of protein fibers, such as actin or amyloid fibers, derived from soluble cytosolic protein precursors.