Water is one of the most important media in the pharmaceutical industry. Due to the wide range of applications of water in pharmaceutical production, its microbiological control is of great importance. In this presentation, water will be explained in more detail as a possible source of contamination and the regulatory requirements for microbiological control of water will be discussed. An interesting insight into the microbial spectrum typical for each purity grade of water will be provided and practical approaches for planning and execution a water hold time study will be demonstrated.