Ultrapure water (UPW) is one of the main materials for electronics fabrication and therefore, it needs to be monitored for critical parameters such as nanoparticles (NP). The state-of-the-art online measurement techniques are challenged by particles at the killer particle sizes smaller than 10 nm. Due to the uncertainties in NP detection, the identification of NP sources and sinks in UPW system is limited nowadays. This review article aims to give an overview on the current developments and perspectives in metrologies for detection and control. The following topics will be discussed: transferability of general definition of NP to UPW, state-of-the-art particle analytics, sources and sinks of NP in UPW systems as well as dominant particle interactions responsible for NP contamination. This article was originally published in the Ultrapure Micro Journal in November 2017.