UGG slippers have been found to contain both wool and linen possibly rendering them shatnez and assur to wear.
A linen reinforcement fabric was found beneath the fleece part of the slipper and a picture was sent to The International Association of Professional Shatnez Testers.
An email was then sent to the customer service of UGGs who confirmed that all their products contain linen.
However, Rav Avraham Granat, a shatnez tester for NCSTAR, says that after testing several pairs of UGGs only some were found to contain shatnez. Rav Granat recommends that all UGGs be checked al pi halacha, just like any item that contains wool.