Multi-impeller stirred vessels are widely used for industrial applications. Based on the numerous studies that reported the motivation and importance of studies on multi-impeller systems, a systematic study was conducted to identify the focus and objectives of research and types of experiments conducted using multi-impeller systems. Researchers mainly focused on the effects of impeller spacing, off-bottom clearance, and type of impeller combinations. Most experiments were conducted on power number, power consumption, gas hold-up, and gas-liquid mass transfer. Research works have not exhausted all impeller-type combinations and there are still opportunities for future work. Computational fluid dynamics studies involving multi-impeller systems are also still lacking owing to flow complexities. This work can serve as a road-map for future study themes.