Worn as the first layer next to the skin, a base layer should be worn all year and changed with the season, with the aim of properly regulating your temperature. From ascending snow capped mountains in scorching temperatures, to ploughing through wintry snow, a base layer is a ride essential.
Popular models can be made from either highly technical man made fabrics which do perform extremely well, or from a light weight merino wool. However, their role is the same, to either keep you cool or warm depending on the conditions whilst pulling sweat away from your body. Within cycling there is much debate over which is the better option, but rather than getting involved in that, our advice would be to wear the material you have historically preferred, and which appeals to you as a cyclist and suits the conditions in which you ride.
If there is one item in your cycling wardrobe who's performance is drastically affected by fit, it is the cycling baselayer. It is simply essential that the baselayer sits directly against the rider's skin where it can work to move sweat away from the body in order to properly regulate core temperature. A baselayer that is loose will simply not function. Keep it fitted, keep it working!