Wearing thermal underwear is a must when fishing cold climates or wading cold mountan streams.
Polyester is the best for thermal underwear as it has less moisture wicking that comes from the perspiration from your body.
When your thermals become wet they draw heat from your body.
Thermal underwear 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.
Thermal underware 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 underwear pants offer some sun protection and thermal protection in mountain streams.
Thermal underwear can be used as thermal layers with other thermal wear in extreme cold conditions.
Thermal underwear can come in a range of diferent fabrics.
Silk thermal underwear, merino wool but I prefer synthetic fabrics like polyester over merino wool.
Good thermal underwear used with other thermal clothing as a outer layer can keep you warm and dry no matter how cold it is.