Just insert batteries and it’s working. The Geiger counter immediately detects particles emitted by a radioactive source. As each particle enters the self-contained probe, an audible click is heard and an LED light flashes. The number of clicks heard over a time interval represents a relative rate of decay of the radioactive material being sampled. The entire unit is enclosed in a rugged, one-piece, molded outer case for total protection of the probe. Comes with convenient carrying case and operating instructions. Uses four AAA batteries (not included).