In this dissertation, I investigate how and why adults differ in the ease with which they learn tone in a non-native language. I examine the extent to which individual variability in tone learning facility depends on factors attributable to a learner’s first language, namely the function of pitch for lexical distinctions (‘L1 tonal status’) and the shapes of native tonal and intonational contrasts (‘tone type’), as well as extralinguistic factors, namely musical experience, working memory, and pitch perception aptitude. In doing so, I aim to provide a novel and integral account of the multiplicity and diversity of factors that influence non-native tone learning facility. The core of this dissertation consists of four empirical data chapters in the shape of journal manuscripts, which each zoom in on non-native tone learning through different lenses. Chapter 1 provides a general introduction. Chapter 2 reports a lab-based study in which 41 Mandarin and English speakers took part in a tone categorization and word identification task to investigate individual variability in pre-lexical and lexical tone perception. Chapter 3 reports two further lab-based studies to investigate pre-lexical and lexical tone processing in the spoken modality to zoom in on individual variability in production. Chapter 4 provides a comparative analysis between the perception and production tasks to discuss differences and similarities between performance in the listening and speaking modalities. Chapter 5 reports a web-based study which involved 114 speakers from typologically different languages (Dutch, Swedish, Japanese, and Thai) and which reassesses the degree to which L1-specific and extralinguistic factors determine tone perception and lexical processing. Chapter 6 provides a general discussion and conclusions. The findings from these empirical studies show that individuals differ greatly in the ease with which they learn non-native tones, particularly at a lexical level of tone processing. Both L1-specific and extralinguistic factors explain why some individuals learn tones with more ease than others do, but these factors interact with one another in dynamic ways to determine tone learning facility. An ‘L1-Modulated Domain-General Account’ is proposed to formally describe the empirical findings from these studies: individual variability in tone learning facility is best captured by extralinguistic factors, but the relative effect of these factors may be modulated by a learner’s language background.