FLASH (free electron laser in Hamburg) is a unique, ultra-brilliant soft x-ray source providing highly coherent femtosecond pulses, currently in a wavelength range of 6.8-47 nm. Up to several 1012 coherent photons within a 10-70 fs pulse allow the study of dynamical changes in nanometer-sized structures. This is a big step towards the ultimate goal of observing femtosecond dynamics on the atomic length scale, for example 'watching bio-machines at work'. In this review, the properties of FLASH are summarized with a focus on coherence, and the potential of FLASH for structural investigations is illustrated with an overview of the recently performed coherent imaging experiments.