This chapter describes the development and validation of an observational measure of collaborative competence that can be used in naturalistic settings. The measure was initially developed and validated through in situ observations within an urban bilingual Head Start classroom. The details of the methods chosen derive from 13 principles for assessing intersubjectivity and collaboration according to the findings and arguments outlined in the previous chapters. These chapters argued for a culturally expansive, emergent, activity-based, and developmentally informed approach to defining collaborative competence. Each principle is elaborated with specific procedures and measures. The chapter discusses ethnographic details of data collection within five urban Head Start preschool centers over the course of a year. The chapter reports on coding of observational measures along with their psychometric properties.