How should I layer under an expedition parka?

The key is avoiding over-layering, which causes sweating that freezes when you stop. Start with a moisture-wicking synthetic or merino wool base layer (never cotton). Add a fleece mid-layer (100–200 weight) for most conditions. For extreme cold, add a lightweight down vest over the fleece. The expedition parka itself provides the equivalent of a heavy mid-layer and outer shell combined. Avoid thick, bulky mid-layers that compress the parka's insulation—you want the parka's fill to maintain its loft for maximum warmth. Adjust by removing or adding the mid-layer, not by changing the parka.