The current study explored the assessment of English Language Learners' argumentative speaking ability. Specifically, the study involved the development and administration of a scenario-based test that aimed to measure learners' ability to display competency in L2 argumentative speaking ability by building an argument from factual evidence and presenting it to a simulated audience. In addition, the study aimed to examine whether embedding assistance in the test can help overcome gaps in background knowledge. Following a mixed-methods approach, results from the test administration were analyzed using robust statistical analyses (e.g., MG-Theory, Rasch Analysis, Multiple Regression) and qualitative analysis of the responses (e.g., rhetorical moves and theme-based analysis). A learning-oriented approach (Purpura & Turner, 2018) to assessment as a design and validation framework was used to interpret qualitative and quantitative evidence. It is hoped that this research project will shed light on the assessment of a foreign language within complex tasks and will help understand the affordances of implementing embedded assistance in language assessment. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]