Synthetic microbial co-cultures can enhance bioprocess performance by division-of-labor strategies that, through spatial segregation of product-pathway modules, circumvent or mitigate negative impacts of the expression of an entire product pathway in a single microorganism. Relative abundance of the microbial partners is a key parameter for the performance of such co-cultures. Population control strategies based on genetic engineering have been explored, but the required interventions may impose an additional metabolic burden and thereby negatively affect co-culture performance. Regulation of co-culture composition by controlled substrate feeding strategies or temperature control requires real-time population monitoring. Process analytical technology (PAT) is an approach for real-time monitoring and control of processes, enabling continuous observation of co-cultivation that may serve as a foundation for population control strategies. In this review, we discuss PAT methods for monitoring synthetic co-cultures, either through direct biomass measurements or by tracking soluble or volatile metabolites. We discuss advantages, limitations, and applications of established as well as emerging technologies and conclude that leveraging PAT for precise, real-time population control has the potential to enhance stability, efficiency, and industrial scalability of synthetic co-cultures.