All normal children in normal environments acquire language. However, all normal children in normal bilingual environments do not acquire two languages. This chapter asks what makes the simultaneous acquisition of two languages more difficult than the acquisition of one. Focusing on children in immigrant families whose two languages are a minority language used more at home and a majority, societal language, this chapter describes common patterns and individual differences in bilingual development. The most frequently occurring outcome in that circumstance is strong skill in the majority language with more varied and weaker skills in the minority language. This chapter also reviews research that identifies factors that contribute to individual differences in order to identify the experiences and abilities that support bilingual development. Those factors include the quality and quantity of children's exposure to each language, children's use of each language, and the functional value of proficiency in each language. We conclude that two languages are more difficult to acquire than one because language acquisition requires substantial and continued environmental support. It is not easy for children to acquire strong and comparable skills levels in two languages because environments tend not to provide high and comparable levels of support for two languages.