Single spacecraft observations of ULF waves can only give limited information on the properties of these waves. One can determine e.g., polarization and use minimum variance analysis to obtain an indication of the propagation direction. To obtain more and more accurate information, it is necessary to use multi-spacecraft data. With the arrival of the Cluster quartet and future multi-spacecraft missions like THEMIS and MMS, there is the possibility to obtain time series at different locations, which can be used to distinguish between spatial and temporal behaviour of the particles and fields. The first part of this paper is a tutorial-review of the techniques that can be used for multi-spacecraft magnetic field data analysis and the second part gives an overview of multi-spacecraft observations of ULF waves in the Earth's magnetosphere, with an emphasis on the recent work by young scientists in the field.