In this paper, we present MedleyAssistant, a system to assist in the creation of music medleys from segments of existing music. Our goal is to make medley creation more accessible to novices, while still allowing them to express their own creative style. Our system addresses two key challenges in medley creation – determining which segments of music sound natural when transitioning to which other segments of music, and determining specific transition points between two given segments of music. This constrains the problem so that medleys created with our system tend to sound natural while allowing the user to be creative with music selection. We also provide a music visualization that helps users understand the musical principles of medley creation.