Layering is the most important aspect of outdoor gear. All-in-one gear lacks the flexibility that you require to stay comfortable. These layers work together to keep you warm, dry, and prepared for any changes in the weather.

Base layer: Wicks sweat away to keep you dry.

Mid‑layer: Traps warmth to keep you comfortable.

Outer shell: Shields you from wind and rain. (See Jackets for more info)

Base layers are your first line of comfort. Worn directly against your skin, they manage moisture by wicking sweat away so you stay dry and warm. A base layer should be lightweight, close‑fitting, and made from fabrics like merino wool or synthetic blends that dry quickly.

Merino wool in particular is mine and many other hikers go to. It can naturally pull moisture away, regulate temperature in both warm and cold conditions, and resist odour for days (something very useful when doing multiday hikes). 

Avoid cotton! Once wet, it stays wet and will make you cold fast.

Mid‑layers keep the engine running. This is your insulation layer that traps warm air close to your body while still letting moisture escape from your base layer. Mid‑layers should be warm, breathable, and easy to adjust as conditions change.

There are 3 main types: fleece, synthetic insulated jackets, and lightweight down.
Fleece is durable, quick‑drying, and works even when damp.
Synthetics give reliable warmth in wet conditions and pack down small. Down is unbeatable for warmth‑to‑weight, but loses performance if it gets soaked unless it has a water‑resistant treatment.

These layers can be mixed and match together with your base layer to help fine tune your temperature. I nearly always carry a fleece and synthetic/down jacket when going out in Ireland.