Ejectors are used for the suction of secondary fluids through momentum transfer and the energy from high-speed primary jets. Ejectors operating with incompressible fluids (liquids) are known as jet pumps or eductors. In other words, the term “ejector” is used when the equipment operates with compressible fluids (gasses and vapors). These phenomena are associated with considerable thermodynamic complexities in the mixer and diffuser. Microscale multistage ejectors are able to reduce losses and increase the efficiency of those MEMs electronic devices requiring liquid pumping. In this thesis, various types of multistage ejectors were studied and then the microscale multistage ejectors were mathematically modeled and simulated with the help of computational fluid dynamics (CFD) techniques. The secondary objective of the present study was to investigate the efficiency of microscale multistage ejectors. Microscale modeling of this type of ejectors is in fact an important aspect of this thesis. The fundamental equations of the model were solved using the commercial FLUENT software package. A reasonably good agreement was observed between the numerical, experimental and analytical results.