Guideline documents are documents that generally clarifies a directives or regulation, as Dr. Simon mentioned, they are not mandatory but they are considered and accepted as the best way to follow a directive. I would have to agree with most of the forum, that NB-MED guidance documents are the most important and useful to follow since they are written by Notified Bodies and they are the central piece of the EU regulatory structure and the ones that pass or fail the device for CE marking. These guideline documents are telling you what the Notified body will be looking for when inspecting a medical device. To me, NB-MED guidelines are the one to follow in order to unsure the CE mark and start selling the medical device in the EU, since failing of the device results in time and money losses as well as more time and money to be spent in order to fix everything for approval and obtain the CE mark.