Spin transfer between spin polarized conduction electrons and magnetizations of the magnetic layers in nanofabriacted magnetic structures is perhaps the most promising mechanism for manipulation of magnetoelectronic devices. Besides the potential practical importance, this effect provides access to the aspects of magnetic dynamics in nanostructures that were previously not experimentally accessible. I will talk about our studies of spin transfer effect in magnetic devices with dimensions not exceeding 100 nanometers. Such small devices usually behave as macrospins. In other words, their magnetization is coherent throughout their volume. I will demonstrate that spin transfer induces very complex nonuniform dynamical states of nanomagnets, which are also extremely sensitive to the minute details of the device geometry and experimental conditions such as the direction and magnitude of the magnetic field. I will show that micromagnetic simulations c apture the most important aspects of data, but some results cannot be reproduced by simulations for any reasonable parameters of the nanostructures, raising important questions about the limits of the applicability for the micromagnetic models, and the possibility of current-induced effects not described by the existing theories of ST.