Mandarin Chinese tones are known to be difficult for second language learners. A large body of research has examined non-native perception of tones, and may provide useful and interesting insights about the sources of tone learning difficulty for Chinese teachers and learners. However, much of the literature is in journals that may be difficult to access or written in technical language that may be hard for non-specialists to understand. This review article aims to summarize key findings from this research in an accessible fashion. I will draw on the research to answer five broad questions: (1) Why are tones more difficult for some learners than others? (2) Why are some tones more difficult than others? (3) Why are tones in words more difficult than in meaningless syllables? (4) Why are tones in context more difficult than in isolation? (5) What can we do about tone learning difficulties?