Considering only the period from November 4, 2006 until the end of 2009, the statistics of the differences between the values of U.Porto ZTD and the corresponding IGS/EPN are (in mm): 0.0 and 4.0, for the mean and standard deviation, respectively. Considering each individual station, the mean difference varies between 2.1 mm and 4.4 mm and the standard deviation between 2.4 mm and 6.8 mm. These values are clear indicators of the precision of the U.Porto ZTD solutions, where the adopted processing methodologies are state-of-the-art in this field. The same applies to all “IGSnew” ZTD and EPN ZTD since November 4, 2006. Before that date, the precision of the online ZTD of most EPN sites degrades to 5–6 mm.