The Development of the Grammatical System in Early Second Language Acquisition focuses on the acquisition process of early L2 learners. It is based on the following key hypothesis: the initial mental grammatical system of L2 learners is constrained semantically, syntactically and mnemonically. This hypothesis is formalised as the Multiple Constraints Hypothesis. The empirical test of the Multiple Constraints Hypothesis is based on a large database including cross-sectional and longitudinal data from square-one ESL beginners. The study demonstrates that the postulated constraints are relaxed successively as learning progresses.