With MER, many novel emotion-based music retrieval and organization methods can be developed. One can search music by emotion, find music pieces that are similar to a given music piece in terms of perceived emotion, or browse a music collection in the emotion space. This chapter describes a music retrieval platform that organizes music pieces in the 2D valence-arousal emotion plane. Such a simple 2D user interface is useful for mobile devices that have a small display space and a limited input capability.