The real-time (or life testing) soft-error rate (SER) measurement is an experimental reliability technique to determine the SER from the monitoring of a population of devices subjected to the natural radiation environment and operating under nominal conditions. This review presentation gives a survey over different real-time SER experiments conducted in altitude and/or underground over the past decade. We discuss the specific advantages and limitations of this approach as well as its comparison with accelerated tests using intense particle beams or sources.