To support future regulation of protozoa in drinking water, the Safe Drinking Water Act Amendments of 1996 require the U.S. Environmental Protection Agency (EPA) to evaluate the risk to public health posed by drinking water contaminants, including waterbone parasites, such as Cryptosporidium and Giardia. To implement these requirements, EPA must assess cryptosporidium and Giardia occurrence in raw surface waters used as source waters for drinking water treatment plants. EPA Method 1623 was developed to support this assessment. EPA initiated an effort in 1996 to identify new and innovative technologies for protozoan monitoring and analysis. After evaluating potential alternatives to the then-current method through literature searches, discussions with research and commercial laboratories, and meetings with experts in the field, the Engineering and Analysis Division within the Office of Science and Technology within EPA's Office of Water developed draft Method 1622 for Cryptosporidium detection in December 1996. This Cryptosporidium-only method was validated through an interlaboratory study in August 1998, and was revised as a final, valid method for detecting Cryptosporidium in water in January 1999.