This thesis examines disyllabic tonal productions produced by thirty Englishspeaking learners of Mandarin Chinese and tests for evidence of three phonological constraints, the Tonal Markedness Scale (*T2>>*T4>>*T1), Positional Faithfulness Constraints (tones at privileged positions have identical values), and the Obligatory Contour Principle (two identical whole tone sequences are prohibited) in the dataset. The tonal grammars of these speakers are accounted for within Optimality Theory, which describes a grammar as a set of universal, ranked constraints. It is shown that these three constraints are all relevant in the dataset. It is argued that these phonological effects result from universal markedness constraints that are present in these learners’ grammars, but are masked in the learner’s target language grammar by the effects of higher ranking constraints. These constraints emerge in the second language acquisition data and represent the situation of “the emergence of the unmarked” in second language phonology.