Miniaturized bioreactors (mini and microbioreactors) have emerged as powerful tools for the study and development of high-throughput microbial fermentation processes. By scaling down cultivation volumes of laboratory bioreactors to the milliliter or microliter range, while still closely mimicking larger bioreactors, these systems allow many controlled fermentations to take place in parallel, drastically increasing experimental throughput by decreasing cost, labor, area, consumables, and time. Recently, significant advances have been made in their design, automation, sensing capabilities, and use across different biotechnology fields. This chapter provides an overview of miniaturized bioreactors, focusing first on their design to discuss the main challenges. It then summarizes key developments with representative studies and industry implementations, followed by insights into future trends.