WaterChek is a proficiency testing scheme for both water chemistry and water microbiology laboratories and is accredited to ISO/IEC 17043:2023 - the internationally recognised standard for competence of proficiency providers. With more than 100 participants in 70 countries, WaterChek has been designed to enable valid comparisons between results from laboratories engaged in testing water for research, industry, environmental or public health testing.  Participants use a wide range of tests and methods, and may aim to achieve quite different limits of detection or analytical accuracy and precision.