The precision of any measurement is limited by quantum mechanics. Yet, in practice, hardly any measurement reaches its quantum limits. This is because typically, dephasing influences the measurement device, rendering sensitivity below its physical limits. A new class of quantum sensors based on spin defects in materials like diamond; however, reach quantum-limited precision even under ambient conditions. Such sensors, e.g. allow for very precise detection of quantities like magnetic and electric fields, temperature and pressure. They have e.g. detected single electron or even nuclear magnetic moments in such complex environment as living cells. In the talk, he will introduce the basic physical concept of the quantum spin sensors and describe recent applications.