This thesis presents the design, development and experimental validation of a novel thermal aerosol generator, designated as UFAG J2, which was conceived to generate stable and reproducible ultrafine particle aerosols under well-controlled operating conditions. The controlled production of ultrafine aerosols is one of the basic needs in the field of aerosol science, environmental research, and in many industrial applications where reproducibility and accurate particle characterization are important. Building on a detailed review of the basics of aerosols and the existing generation methods, which include condensation-based, atomization, powder dispersion, and combustion generation methods, this work provides motivation to choose thermal aerosol generation as a suitable method for the production of ultrafine particles with size distributions that are representative of urban and indoor aerosols.