The sequence of experiments resulting in the development of steady-state dense hot-electron plasmas is briefly described. These plasmas are produced in magnetic-mirror machines by radiation at the electron-cyclotron frequency. The electron-cyclotron plasma with the greatest stored energy to date has a volume of ∼ 50 liters, an electron "temperature" of 120 keV, a density of 4 — 7 × 1011/cm3, an d an average beta of ∼ 0.4. This plasma is created in the EPA Facility by a 50-kW c.w. 10.6-gHz microwave power source. The construction and operation of the machine are briefly discussed. The analysis and interpretation of the bremsstrahlung and diamagnetic measurements employed to determine these parameters are given. Diamagnetic and particle decay measurements on a smaller machine, the Physics Test Facility, are also described.