What factors determine or contribute to bilingual speakers' proficiency, or their overall knowledge of and fluency in each of their languages? This chapter highlights the multiple factors influencing proficiency. This discussion adopts the position, from the start, that proficiency should be kept separate from language ability (Peña, Bedore, & Fiestas, 2013); the latter has to do with the language-learning capacity itself, whereas the former refers to a speaker's sum-total knowledge and facility in using the particular language in question. This distinction is important because it is often difficult to tease apart level of proficiency in a language from language ability, and that is an issue that speech and language therapists, educators, and parents must cope with. The remedies for difficulties in ability and proficiency can be drastically different (see, e.g., Gathercole, 2013aGathercole, , 2013b. When the distinction is not recognized, bilingual children may be misdiagnosed as having speech problems (abilities) and be sent for speech therapy simply because they are struggling with a second language (L2 [proficiency]; Letts, 2013; Peña, Bedore, & Fiestas, 2013), or they may be overlooked for speech problems because of implicit assumptions sometimes made about acquisition in bilingual speakers (O'Toole & Hickey, 2013). Kindergartners or young grade schoolers whose first language (L1) is distinct from the dominant language of the classroom may be viewed as academically inferior, again simply because an inability to navigate the academic subjects in the L2 (because of a low level of proficiency) is interpreted as an inability to handle the content of the material (because of low ability). This chapter takes the position that for normally developing bilingual speakers, we can assume that language abilities are commensurate with those of monolingual speakers. Proficiency, on the other hand, as observed in performance in the language-specific use of vocabulary or grammar, may vary from one child to another or one group to another because it is a product, in monolingual and bilingual speakers, of one's range of experience, exposure, and use of the language in question.The purpose of this chapter is to examine what factors affect and modulate the language proficiency observed in bilingual children and adults. It will lay out the contributions of the quantity and quality of input in the home and in the community, and the role of the sources of that input; discuss the distributed characteristic, profile effects in bilingual speakers, and the role of linguistic complexity; touch on inter action between