Collaborative problem-solving (CPS) skill is an important 21st century skill (Griffin, McGaw, and Care, 2012). However, assessing CPS, particularly in a standardized way, is challenging. The type of collaboration, size of teams, and assessment domain all need to be properly considered when developing a CPS assessment. In this chapter, we outline some practical challenges for developing a large-scale, standardized assessment for CPS and present some strategies to address those challenges. We illustrate these strategies with the Collaborative Science Assessment Prototype (CSAP) developed at Educational Testing Service.