When the first discussions on polarization mode dispersion (PMD) started in 1986 , only a few people considered that this effect might become part of their daily business. Since then the Internet, broadband services, etc. have generated a bandwidth demand that drives high-speed transmission at rates of 10 Gbit/s and above. While research on polarization mode dispersion goes on for the next generation networks [2, 3], it is essential to know how PMD influences today's network performance. Understanding the causes of PMD, and the advantages and disadvantages of the different measurement methods that are available for field-testing, are key for building reliable and profitable high capacity networks. Along with chromatic dispersion (CD), polarization dependent loss (PDL) and loss  polarization mode dispersion is an important piece of the puzzle for today's fiber certification. Part II of our Series on fiber certification will help to understand the issues related to PMD.