A baselayer is worn next to your skin to manage moisture and help regulate comfort. Its main job is not simply to add warmth—it moves sweat away so it can evaporate, helping reduce that clammy feeling during hiking, running, and other active days. In hot weather, a lightweight moisture-wicking top can also serve as your only layer, especially when it adds UPF sun protection.

Synthetic fabrics usually dry quickly, feel durable, and work well for high-output activities. Merino wool naturally manages odor and stays comfortable across a wide temperature range, making it useful for travel and multi-day adventures. Blended fabrics aim to combine these benefits. Avoid cotton for demanding or cold outings because it holds moisture and can feel cold once wet.

Focus on fabric weight, moisture-wicking performance, drying speed, breathability, and the conditions you expect. Lightweight layers suit warm weather and intense activity; thermal or heavier fabrics add warmth in cooler conditions. For exposed summer routes, compare UPF protection, coverage, and ventilation. UPF 50+ blocks at least 98% of UV radiation, but sunscreen is still needed on uncovered skin.