We introduce so-called 'magnetic force response theory' which has been suggested and formulated 1980s and is still under active development. Combined with first-principles electronic structure calculations, this theory becomes such an useful theoretical tool that many of complex magnetic materials and their intriguing phenomena have been unveiled through the unique information it provides. Here we introduce the basic idea and formalism of magnetic force theorem and discuss some of recent developments including the extension to anisotropic interactions and to correlated electron materials. The current status of the related research activities and the available codes are outlined.