Recent progress in the theory of collective pinning of flux lines in type-II superconductors is outlined: Its correction due to nonlocal theory of elasticity, its extension to films of finite thickness which yields the condition for dimensional crossover, and improved computersimulations. The observation of an abrupt reversible transition to a state with highly disordered flux-line lattice is commented. The meaning of the range and strength of pins in a periodic flux-line lattice is discussed.