In this paper, we describe the design and development of a dual-energy system used for x-ray screening of airport carry-on luggage. Dual-energy x-ray systems make it possible to measure the average atomic numbers of screened objects to enable their classification into three categories: inorganic, organic and mixed materials. Detection of organic materials, usually associated with dangerous compounds, mainly plastic explosives, is easier to achieve with dual- energy, as opposed to single-energy systems. The theory behind dual-energy systems is presented, followed by the design of a system based on a sandwich transmission detector arrangement with all its components, such as x-ray detectors, filter, operating tube, etc., and associated parameters are estimated according to simulation data. The process of generating the Z image, which includes the atomic number information from the two base images, is also described. After the prototype had been built, the unit was calibrated and images were taken with materials of known atomic number. Based on those measurements, the unit was tuned for optimal performance. Results comprise all decoding and compression tables for generating the images. Samples of Z and RO images taken from the unit are included and described in the report.