How do I know what mid-layers to wear under my shell?

Start with a moisture-wicking base layer (merino wool or synthetic) against your skin, never cotton. Add a mid-layer based on temperature: a lightweight fleece for warm days (around -2°C to +5°C), a heavier fleece or thin puffy for moderate cold (-10°C to -2°C), and a thick puffy or layered fleece for very cold days (below -10°C). You can always add or remove a layer at lunch. The key principle is that each layer should be easy to add or remove, and your base layer must wick sweat away from your skin.