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If most of us look in our closets and drawers, we will find that much of out clothing is made of cotton (jeans, underclothes, t-shirts, ...). Unfortunately, cotton is not a good material for backpacking clothing. It absorbs and holds sweat too near the body. To make matters worse, cotton loses almost all its insulation value when it is wet, and since it holds water, it takes a great deal of time to dry out. This does not mean that you have to go out and buy a separate wardrobe for backpacking. Certainly, many people have gone backpacking in cotton jeans and t-shirts; however, it pays to remember cotton’s limitations when you are selecting clothes for your trip. If you have t-shirts and sweatshirts which are made of synthetics, wool or blends, these will make a better selection than the same articles of clothing made of cotton. The one article of clothing to pay special attention to is your socks: if at all possible, do not use cotton socks. If you must, be sure to wear a thin pair of synthetic liner socks to keep the sweat away from your skin.

While some people will find wool somewhat itchy, and others may consider it old-fashioned, wool and wool/synthetic blends are very serviceable for backpacking. Wool tends to absorb less sweat than cotton, but if it should become wet, it retains its insulation value. Wool is much warmer than cotton and as warm as - or warmer than - many synthetics. Wool and wool-blend socks are a good choice for outer socks. 

Most synthetic fabric is "hydrophobic"; that is, it tends not to retain moisture from perspiration or precipitation. Instead, it transports the moisture away from the skin. This makes most synthetics (provided that they offer insulation value) a suitable choice for backpacking clothing.