Abstract: In this review we describe the aerodynamic problems that must beaddressed in order to design a successful small aerial vehicle. The effects of Reynoldsnumber and aspect ratio (AR) on the design and performance of fixed-wing vehicles aredescribed. The boundary-layer behavior on airfoils is especially important in the designof vehicles in this flight regime. The results of a number of experimental boundary-layerstudies, including the influence of laminar separation bubbles, are discussed. Severalexamples of small unmanned aerial vehicles (UAVs) in this regime are described.Also, a brief survey of analytical models for oscillating and flapping-wing propulsionis presented. These range from the earliest examples where quasi-steady, attached flowis assumed, to those that account for the unsteady shed vortex wake as well as flowseparation and aeroelastic behavior of a flapping wing. Experiments that complementedthe analysis and led to the design of a successful ornithopter are also described.