Recent progress in magnetic fluid research and technology is reviewed. Remarkable advance in preparation of magnetic fluid is achieved by using iron-nitride magnetic particles whose magnetization shows 3 ~ 4 times higher than those in conventional magnetic fluid. As the topic of the hydrodynamics of magnetic fluid, the pipe flow problems such as gas liquid two-phase flow and oscillatory pipe flow and oscillatory pipe flow are considered by taking into account the particle aggregates in the strong magnetic field. Finally, the development of technology and commercial applications of magnetic fluids are explained. Especially, new technological developments are introduced to spread the application field of magnetic fluids.