The CW21 is a designation from the American watchmakers clockmakers institute.  It stands for 21st century certified watchmaker.  It is given upon the successful completion of the CW21 examination- this is a very rigorous 4 day examination that includes a written 23 page examination of short answer watch related questions.  Then a complete service of a automatic chronograph, a complete service of a automatic watch, a complete service of a quartz watch and a balance staff replacement in a mechanical watch.  All of these watches have specific errors introduced into them and all errors must be corrected. The watches have to be cleaned, relubricated and run to the manufacturers specifications.  All of these must pass according to a specific grading criteria.  The reason it is so important is because of the rigorous nature of the test and the candidates that pass and are awarded the CW21 designation are of the highest caliber of watchmakers. Certain manufacturers do not allow their parts to be sold to watchmakers that do not hold the CW21 designation.  Since there are only about 250 CW21 watchmakers out there and approximately 10,000 watchmakers that are doing service on watches it is very important for you to look for the CW21 designation.  However, the consumer must be aware that since the designation is so recognized in the industry that there are people out on the Internet that claim they are CW21 or have someone working for them that has a CW21, the long and short of it are if they will not tell you who the CW21 is then you should assume they do not have the designation.