Non-tonal languages (NTLs) are 'deaf' to tones unless they are trained to hear them, and this training ought to include a correct categorization. Differently from the case of segmental properties such as consonants, categorical perception of tones is still a relatively open issue. The most convincing hypothesis is that of quasi-categorical perception, but the conspicuous literature on how non-tonal language speakers (NTLSs) process tones differently than tonal language speakers offers important insight to the issues related to tone acquisition. Lexical tones are "pitch patterns that serve to provide contrasts in word meaning". The chapter provides Yang's findings about the overlapping areas in the tone perceptual space of NTLSs, found both in perception and production. As pointed out by Yang in her studies on perception and production of Mandarin tone categories, data on non-native speakers' perception show that "there are no clear boundaries and tones overlap to a great extent over the entire perceptual space".