This chapter focuses on behavioral evidence concerning acquisition of unbounded dependency constructions and island phenomena. It provides a general overview of their acquisitional trajectory in normal developing L1, focusing on the emergence of filler-gap dependencies from one-word constructions to multi-clausal constructions, as well as island effects. The chapter argues that the gradual and frequency-based developmental evidence is consistent with an exemplar-based approach that contains rich morphosyntactic, semantic, and pragmatic information along the lines of the empiricist perspective. In particular, the extant evidence supports a usage-based view in which the growth of a grammar proceeds from simple units to more complex ones, exploiting the frequency of simple as well as complex exemplars. The chapter concludes by describing a exemplar- and chunk-based account of grammar development, composed of rich information that is probabilistic in nature, and shaped by experience.