Base Layers are incredibly important pieces of cycle clothing, designed to be worn next to the skin - base layers not only provide insulation and keep you warm, but they also play an important role in regulating body temperature. Base layers wick sweat away from your body, meaning that sweat is removed from your clothing - keeping you warm in the winter and cool in the summer.
There are many different base layers available, and most of these base layers can be used regardless of cycling discipline - so mountain bikers and road cyclists alike can benefit from them regardless. For a base layer to work properly, it should be tight enough to feel snug - but it should also move freely. There are a range of base layers available, including merino wool base layers from companies such as Mons Royale.