Scholarship and policy emphasize strengthening the early childhood education (ECE) workforce, but this work neglects a large segment of the workforce: assistant teachers. The present study responds to gaps in knowledge by examining the demographic characteristics, qualifications, professional supports, and workplace experiences of assistant teachers (N = 120) in a representative sample of ECE centers (n = 35) in a large urban district. In addition to studying assistant teachers' receipt of in-service training and coaching, we draw from social network theory to investigate the professional support assistant teachers provide and receive via their collegial networks. We use a variance decomposition approach to explore how on-the-job supports, such as training, coaching, and networks, contribute to assistant teachers' work-related stress and job satisfaction--two key predictors of ECE teacher attrition. Results indicate that few ECE staff members seek assistant teachers for work-related advice. Coaching is found to be an important contributor to assistant teachers' job satisfaction; professional advice via collegial networks is a meaningful but under-examined source of support for stress and job satisfaction. We consider implications for supporting and retaining assistant teachers and propose next steps for research on this understudied segment of the teaching workforce. [This paper was published in "American Journal of Community Psychology" v63 p312-323 2019.]