This study investigates factors that affect the accuracy of tone production by American students of Vietnamese as a second language (L2). Nine hypotheses are examined, each of which isolates a factor expected to affect production accuracy: (a) task type, (b) the position of a tone in a clause, (c) discourse distance between a model provided by a native speaker and the learner's utterance, (d) markedness relationships of the tones, (e) emphasis, (f) the preceding and following tones in the string, (g) voicing of the initial consonant, (h) postvocalic elements in the syllable, and (i) vowel height. The findings show that the patterns of tone production by the American learners of Vietnamese are influenced by universal mechanisms of phonetics and phonology, first language structures, and L2-specific rules. The results also provide a framework for determining the difficulty of tones in different environments.We would like to thank the students and teachers at the Southeast Asian Studies Summer Institute at the University of Wisconsin-Madison for their participation in this study. We thank Richard Young, the late Craig Chaudron, and the anonymous SSLA reviewers for helping us strengthen this paper. We are responsible, however, for any errors that remain.