The nocturnal activities of bats have largely eluded observation until now. Recent technological advancements have made it possible to convert the inaudible ultrasound emitted by bats into audible sound using ultrasound detectors. This allows for the observation of bats' lives even in complete darkness. This is not only an impressive experience, especially for young people, but it also opens up opportunities for new scientific insights. This book primarily aims to introduce detector technology while also providing knowledge about bats, such as identification features, distribution, habitat, migrations, threats, and conservation. It explains in detail how bats can be observed with a detector with minimal effort. The physical principles and potential applications are also discussed.