SCRUPULOUS CLEANING IS THE MOST FUNDAMENTAL fire safety measure that can be applied to oxygen systems. The presence of contaminants in otherwise-robust oxygen systems can lead to catastrophic fires. To reduce the hazard of ignition, components and systems should be initially clean and should be maintained clean during assembly and maintenance. The intent of this chapter is to provide a general overview of cleaning for oxygen systems that can be used alongside other cleaning standards. This chapter begins with an overview of cleaning, followed by descriptions of cleanliness levels, cleaning safety, cleaning methods and aids, and cleaning procedures. This chapter is concluded with guidelines for maintaining cleanliness during the assembly and maintenance of components and systems.