Ok, Euro basic material is 100% cotton. Then they add metal, plastic, special inks and whatnot:
* Tactile relief print (not only for security but also helps visually impaired)
* Watermark (visible)
* Watermark (digital pattern, some image processing software refuses to process images containing that digital watermark pattern)
* Security thread with embossed text
* Bar code
* Magnetic ink for some parts
* See-through register
* Foil strip (lower denominations) or foil patch (larger denominations) embedded in the paper. Both with 3D holographic effect
* Iridescent stripe (lower denominations) or optically variable ink (larger denominations)
* Microprinting
* Ultraviolet properties
* Infrared properties
* Starch-free material (most normal paper contains starch, allows for a starch test)
* EURion constellation (a special pattern some color photocopiers recognize and refuse to copy)
* Since a few years rumors about RFID chips in the largest denominations 200 € and 500 €. The European central bank is careful not to confirm or deny the existence of the RFID chips. What they have confirmed is that they have an RFID project since 2001 which had the target of getting the RFID chips into the notes until 2005.
* More secret security features. It is believed to be over 30 features in total.
And still, counterfeiting is on the rise.