The difference in process and product of child (L1) , as opposed to adult (L2), language acquisition, is a topic that has been widely discussed and continuously debated, often in terms of the Fundamental Difference Hypothesis (Bley-Vroman 1990, DeKeyser 2000). This proposal claims that child L1A and adult L2A are essentially distinct in terms of initial state, process and outcome, and it suggests that the reason for this is the availability of UG for the former but not the latter. The adult / child difference is closely linked to a second hypothesis, that of the Critical Period (Lenneberg 1967, Bialystok 2002) proposing that first language acquisition is biologically determined to be inevitable, inexorable and chronologically delimited to a sensitive period during the childhood years. The proposal has spawned ample investigation and controversy between those claiming adult L2A to be abject failure and those pointing to adults who gain native-like proficiency. This talk reexamines the Critical Period Hypothesis (CPH) to sort out the biological from the non-biological. It argues that there is indeed a sensitive period for the acquisition of L1 physiologically "hard-wired" abilities (e.g. phonological, processing), for the operation of involuntary acquisition, and for complete mastery of certain features. It also argues that this sensitive period is not absolute or irreversible and has no defined terminus. In other words, the period has soft limits, not hard ones. It first presents the background on the CPH; then it contrasts process and product in L1A and L2A; and it concludes with a discussion of the author's research in L2A by children learning immersion Spanish.