Sizing info and tag inside the collar have worn away but this fits like a small - medium. If you’re a medium it will fit like a base layer should, close to the skin. If you’re a small it will fit a little looser. This shirt is in great condition and kept me warm on many an outdoor adventure. Description from Duckworth website: The Polaris Crew Merino Wool Shirt is ideal for the outdoor adventurer and do-it-all personality. Duckworth's proprietary Polaris fabric is designed to trap warm air in the miniature waffle knit construction, creating a lightweight yet incredibly warm layer. The super soft waffle construction is meant to be worn directly on the skin, making the Polaris Crew the ideal crossover piece. On hotter days, wear it as a stylish and comfortable solo layer, or on colder days use it as a warm and cozy baselayer. Odor-free, itch-free, moisture wicking, performance-driven, and versatile, this Montana-grown Merino Wool shirt looks just as good on a night out as it does on a mountain's summit. American-made with Montana wool.