This paper reports on a 26-month longitudinal study of a Korean-speaking child acquiring L2 English in the United States (AoA: 3;6). Focusing on negation, plural, and possessive marking, the paper examined the nature of language transfer as a function of changes occurring in the participant’s L1 and L2. It subsequently found bidirectional transfer in the child’s placement of the negator, reverse transfer in certain plural constructions, and a delay in the acquisition of possessive marking that is attributable to the lack of a corresponding feature in the participant’s developing L1. Importantly, this pattern of transfer appears to arise from the waxing and waning of the child’s L1 and L2. Foster-Cohen (2001) suggests a sliding-window approach to understanding child second language acquisition (SLA). She views development as a continuum along a variety of axes, including but not limited to age, cognitive maturity, and proficiency, which are interrelated such that waxing along one axis may coincide with waning along another. Adopting the “sliding window” notion as its conceptual backdrop, this chapter reports on the longitudinal study focusing on language transfer as a function of changes occurring in the child’s L1 and L2. Transfer was herein defined as a process in L2 acquisition (L2A) whereby one language influences the other. This is dubbed substratum transfer when the influence comes from the L1 and reverse transfer when the influence comes from the L2 (Odlin 1989).