Base layers come into their own when managing moisture on summer hikes. Summer base layers are thin and lightweight, designed to pull sweat away from the skin and dry quickly to minimise wind chill.
Merino wool base layers are naturally softer, more insulating and odour-resistant - perfect for multi-day hikes. Merino reacts better with varying temperatures, so you'll still be comfortable if the temperature drops.
Synthetic base layers, like polyester or polypropylene, are naturally stretchy, fast-drying and sweat-wicking, so if you're getting active on a high-speed hike or trail run, or have a naturally higher body temperature, these are the ones for you.
Base layers made from a blend of merino and synthetic fibres will have a combination of these qualities in different proportions, so consider your normal body temperature and your activity to work out which is best for you.