This book discusses the nature of optionality in second language grammars and the indeterminacy observed in second language users' linguistic representations. For these purposes, experimental data from 213 learners of German and 150 learners of Russian have been collected and analysed with a special focus on the acquisition of various "subjectless" and impersonal constructions as well as argument licensing. Whereas voice alternations and argument licensing are topics amply discussed in theoretical domains, their practical implementation within second language research has remained a research lacuna. This piece of work intends to fill the gap.