The language continuum is defined by the dimensions of language ability (from language talent to language disorder) and bilingual language proficiency (from monolingual to multilingual) in this chapter. Although the terms “language ability” and “proficiency” are often used interchangeably in the literature, we systematically differentiate these two dimensions in order to explain variations in speakers from different linguistic backgrounds. Cross-linguistic data are presented to strengthen this argument. The differentiation of these two language dimensions is important for professionals to prevent the over- and underdiagnosis of children who come from language minority groups or exhibit bilingual language disorder. A discussion of similarities in language acquisition between typically developing monolingual and bilingual first language speakers and between monolingual and bilingual children with developmental language disorder is included to highlight common features along the language dimensions. Within-subject approaches and different measures of assessment are described to support the classification of children from diverse linguistic environments.