“Bio Fluids” have the remarkable property of rapidly degrading in natural environments. Unfortunately, the same fluid chemistry leading to biodegradation also leads to fluid instability in machine environments. This dilemma can be resolved through vigilant contamination control. Maintaining biodegradable fluids at extremely low levels of particulate contamination and extremely low levels of moisture discourages fluid deterioration, especially the breakdown mechanisms of oxidation and hydrolysis. Maintaining fluids clean and dry also achieves the additional benefits of limiting the wear and corrosion of mechanical components. The challenge is to control these contaminants in the fluid systems of heavy machinery operating in wet and dusty environments. This paper reviews the effects of contamination on biodegradable fluids and the machines using them, suggests recommended upper bounds for water and particulate contamination, and discusses methods for achieving and monitoring these preferred levels of fluid cleanliness and dryness.