Thermal under garments are a must when fishing cold climates or wading cold mountan streams. Polyester are the best as they don't wick the perspiration from your body and become wet, as when this happens it draws the heat from your body. Shirts should be long in the body and have a longer back to keep you warm and dry. They should be a snug fit but not to tight. The pants should have a wide waist band that doesn't cut into your waist. I use the pants under a pair of shorts when wet wading in hot weather the thermal pants offer some sun protection and thermal protection in mountain streams.