guesteditorial continued an achievement that can impact the emerging field of quantum sensing. Finally, in " Design of an FBG Based Water Leakage Monitoring System, Case of Study: An FBG Pressure Sensor, " Vincenzo Marletta presents an optical fiber multi-sensor measurement system based on FBG sensors, focused on a measurement system developed to measure the main quantities (pressure, flow rate, and vibrations) useful for the detection of water leakages. Overall, we hope that the set of works presented in this special issue will give the reader an insight on the diversity, capabilities, and potential of this vibrant field of optical measurements. 4 IEEE Instrumentation & Measurement Magazine August 2021