An overview is presented of the properties and effective thickness of the interphase formed between fibres and polymer matrices. Chemical and physical characterization of the interphase is discussed to portray molecular interactions comprising the interphase layers in silane-treated glass-fibre composites. The gap between physico-chemical investigation on one side and bulk material testing on the other side is bridged by implementation of novel techniques, such as nanoindentation, nanoscratch tests, and atomic force microscopy (AFM), which have been successfully used for nanoscopic characterization of the interphase in the past few years.