The brushless Doubly-Fed Induction Machine (DFIM) provides an interesting alternative to the commonly applied conventional DFIM in modern multi-MW wind turbines. This paper presents a literature survey on the brushless DFIM. The evolution of the cascaded induction machine into the modern brushless DFIM is discussed, as well as several theories explaining this machines complex operating principles. Nowadays, brushless DFIM research is mainly focussed on the development of a commercially competitive brushless DFIM based drive for multi-MW wind turbine applications. An overview of the latest developments in the fields of brushless DFIM modeling, design, and control is provided. Based on this, a qualitative comparison is presented of the brushless DFIM and other well-established generator systems for wind turbines and the challenges for future research are explored.