Gas-filled detectors have always been widely used because of their simplicity, being proportional counters the most indicated for making spectroscopic studies as well as for counting. This book is focused on a particular proportional counter: a windowless gas flow, which presents the particularity that the source is directly introduced in the counting chamber. The procedure of its full setting-up, including the choice of geometry, the wires, the required nuclear electronics and the optimal parameters of working, as well as the results analyzed for several alpha decay emitters and a few beta samples, are described. The understanding of the working of the detector and the learning of its optimal use are reached in an exprimental way through addressing all the difficulties found in its tune-up, beginning from the construction of the device.