Recent developments in low-temperature detectors incorporate genuine quantum features, promising to reach unprecedented levels of measurement precision. On a fundamental level, these highly sensitive devices will give us a glimpse into the world of quantum/thermal fluctuations and the effect of measurement backaction, and will advance the utilization of quantum resources and feedback. They will create a pathway to scientific breakthroughs: novel phenomena in condensed-matter physics, enhanced spectroscopical characterization, observation of new particles, and understanding of the interaction between gravity and quantum-mechanical objects.