In this article, we aim to contribute to the debate about the use of subordination as a measure of language proficiency. We compare two theories of SLA—specifically, processability theory (PT; Pienemann, 1998) and dynamic systems theory (DST; de Bot, Lowie, & Verspoor, 2007)—and, more particularly, how they address the development of subordinate clauses. Whereas DST uses measures from the complexity, accuracy, and fluency research tradition (see Housen & Kuiken, 2009), PT uses the emergence criterion to describe language development. We focus on the development of subordinate clauses and compare how subordination as such is acquired and how the processing procedures related to a specific subordinate clause word order are acquired in the interlanguage of second language German and Swedish learners. The learners’ language use shows that the use of subordination (as measured by a subordination ratio) fluctuates extensively. From the beginning of data collection, all learners use subordinate clauses, but their use of subordinate clauses does not increase linearly over time, which is expected by DST. When focusing on processability and the emergence of subordinate clause word order, however, a clear linear developmental sequence can be observed, revealing a clear difference between the nonacquisition and the acquisition of the subordinate clause word order rules. Our learner data additionally reveal a different behavior regarding lexical and auxiliary or modal verbs.