Professional learning communities (PLCs) have emerged as one of the nation's most widely implemented strategies for improving instruction and PK-12 student learning outcomes. PLCs are predicated on the principles of improvement science, a type of evidenced-based collective inquiry that aims to bridge the research-practice divide and increase organizational capacity to solve pressing problems of practice. In this article, the Teacher Collaboration Assessment Rubric (TCAR) is presented in which the evidenced-based attributes of rigorous PK-12 PLCs are operationalized. The author describes how the TCAR has been used for developmental, formative, and outcome evaluation purposes in PK-12 settings. The value of the TCAR and the evaluation of PLCs in other complex organizational systems such as health care and the sciences are also discussed. PLC evaluation can help teams to focus on improvement and to avoid "collaboration lite" whereby congeniality and imprecise conversation are confused with the disciplined professional inquiry vital to organizational improvement.