When you layer for skiing, you will need at least three layers. On a cold, windy day, you'll need four. Your breathable base layer and waterproof breathable outer layer are the two layers you should always wear when skiing. You can wear one, both or neither of the mid-layer and insulating layer, depending on the conditions. The colder the temperatures, the more layers you'll need.
Here's a step-by-step guide on how to layer for skiing:
1. Your first layer should be a moisture-wicking, breathable and midweight base layer made of merino wool, polyester or silk. This layer will wick moisture away from your skin and allow excess body heat to escape, as well as provide your first layer of warmth. You should have a base layer top and bottoms.
2. The second layer is a mid-layer. Depending on if you have three or four total layers, your mid-layer may change. Like the base layer, the mid-layer should be breathable and moisture-wicking. It also acts as a source of low-bulk warmth. Polyester and merino wool pullovers are both great mid-layer options for skiing. You should have a mid-layer top, but mid-layer bottoms aren't usually necessary.
3. The third layer is an insulating layer. This is the layer you can do without if you're skiing in warm, sunny conditions (think spring skiing) or are wearing an insulated ski jacket. During most of the ski season, you'll need this layer to provide warmth. If it's very cold or windy, you may still need a third layer even if your ski jacket is insulated. Fleece jackets, heavy wool sweaters, down-insulated vests and jackets are the best insulating layers. If you have insulated ski pants, you don't need insulated layer bottoms.
4. The fourth, or outer, layer acts as your shield from the elements. This final layer should be waterproof breathable, or at least highly water-resistant and breathable. You'll need this outer layer for both top and bottom. The ski jacket and ski pants may or may not be insulated. Many skiers wear a waterproof breathable, insulated ski jacket and pants or a protective shell over an insulated layer.