A liquid-to-air membrane energy exchanger (LAMEE) is a novel flat-plate membrane-based energy exchanger where heat and moisture transfer between air and solution streams occurs through a semi-permeable membrane. The LAMEE consists of many air and solution flow channels, each separated by a membrane. A small-scale single-panel LAMEE consists of a single pair of neighboring air and solution channels. This PhD thesis focuses on developing, testing and modeling the small-scale single-panel LAMEE, and investigating the similarity between the small-scale LAMEE and a full-scale LAMEE. This PhD thesis presents a methodology to investigate similarity between small-scale and full-scale energy exchangers. A single-panel energy exchanger test (SPEET) facility is developed and built to measure the performance of the small-scale single-panel LAMEE under different test conditions. Also, the small-scale LAMEE is numerically modeled by solving coupled heat and mass transfer equations for the air, solution and membrane of the LAMEE. The effects of membrane vapor diffusion resistance and enhanced air side convective heat transfer coefficient are numerically investigated. The numerical model of the small-scale LAMEE is validated with the experimental data for summer test conditions, and effectiveness values agree within ±4% in most cases. Moreover, the effects of different heat and mass transfer directions, and salt solution types and concentrations are experimentally and numerically investigated. The results show that the LAMEE effectiveness is strongly affected by the heat and mass transfer directions but negligibly affected by salt solution type and concentration. The solution-side effectiveness for liquid-to-air membrane energy exchangers is introduced in this thesis for the first time. The results show that the solution-side effectiveness should be used to evaluate the sensible and total effectiveness of LAMEE regenerators. Finally, the similarity between the small-scale and full-scale LAMEEs is investigated experimentally and numerically. The results show that the small-scale LAMEE effectiveness results can be used to predict the performance of a full-scale LAMEE within ±2% to ±4% in most cases.