First and foremost, consult a knowledgeable boot fitter when you purchase ski boots. Beyond being uncomfortable, poorly fitting ski boots can be dangerous in the backcountry.
Overland ski touring is somewhat akin to summer hiking, so boots for overland ski touring are quite similar to hiking boots. Fit these boots for comfort and consider warmth when selecting a boot.
Most overland touring boots are now equipped with an integral binding system. See the discussion of these bindings systems in the binding section of this FAQ for more details.
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Shortly after the telemark turn was popularized (again)in the 1970's, telemark boots were similar to heavy hiking boots or were converted from old leather downhill ski boots (called Steincomps). Since that time, some telemark boots have remained virtually unchanged from their hiking boot origins, and others have undergone radical transforma- tions. The latter boots include models with a full plastic buckled shell (similar to alpine ski boots), models with an upper plastic cuff that is buckled and a lower laced leather boot, models with a lower plastic shell but are fully laced and models with plastic underneath the leather exterior.
There are two big differences between plastic and leather boots. Generally the more plastic a boot has, the better that boot is at controlling the ski during a turn. On the other hand, plastic boots are usually less comfortable and weigh more than their leather counterparts.
Most telemark boots are constructed with a 75mm nordic-norm toe that uses a three-pin or cable style binding. Some telemark boots are constructed to use with the NNN-BC (New Nordic Norm - BackCountry) bindings. NNN-BC boots and 75mm toe boots are incompatible with each others bindings. The primary advantage of the NNN-BC system is that the boots are generally lighter than their 75mm counterparts. The 75mm toe is used on most heavy telemark boots designed for downhill skiing. See the binding description in this FAQ for more information on the 75mm vs. NNN-BC binding systems.
When deciding which boots to purchase, first and foremost, only buy boots that fit. Second, you must decide how much plastic you need. If your primary backcountry skiing activity is a hike in the mountains, then you should opt for lighter (most likely leather, possibly NNN-BC) boots. If your primary skiing activity is telemarking on steep terrain, then you should opt for the control of stiff boots.
Annual reviews of telemark boots are available in magazines such as Backcountry, Couloir, and Powder.
Alpine touring boots are constructed with a plastic shell and a removable inner lining. The soles of these boots have a "Vibram" style lug, they are rigid, and they are designed to be used with Alpine Touring bindings only. Some AT boots are designed more for walking comfort while others are designed more for skiing control. A good salesperson that has several models of AT boots should be able to explain the differences.
Since AT boots have a rigid sole, a proper fit is critical to have a comfortable boot. AT boots should be fit with more room than alpine boots, particularly in the ball of the foot and the toe box. Yet they should still be snug to prevent blisters. Remember you are going to walk in these things, not just ski in them.
Annual reviews of AT boots are available in magazines such as Backcountry, and Couloir.
As with alpine skis, alpine boots are regularly used in the backcountry. For an overview of alpine boots, see the rec.skiing FAQ. Generally alpine boots that are comfortable when the buckles are open are best for backcountry skiing because during the ascent many people leave their buckles undone. The two biggest disadvantages of alpine boots are that they are heavy and uncomfortable when walking.