Learners acquiring a second language (L2) face a similar challenge as first language (L1) acquirers in that the properties of grammar are complex, abstract, and not fully determined by the L2 input. This suggests that principles of Universal Grammar (UG) constrain interlanguage grammars, analogous to how UG principles constrain L1 acquisition. While L2 learners may not achieve the same level of knowledge as native speakers, they can still demonstrate poverty-of-the-stimulus situations where their grammars are influenced by UG principles rather than direct instruction or exposure alone. The evidence presented in this chapter supports that interlanguage grammars are constrained by principles of UG.