Shelter

Out of the four basics requirements for survival (fire, food, water and shelter) shelter comes out pretty much at the top of the list irrespective of the environment you are in.
Hypo and hyperthermia are real and serious risks to people in remote areas, and should never be underestimated. The human body gains or loses heat three ways; radiation, convection and conduction.
Radiation is where either your body heat is radiating away to the cooler surrounding air or you are absorbing heat from the direct or reflected heat of the sun.
Convection, this is where air currents either remove the trapped layer of warmer air around you or remove the cooling effect of sweating.
Conduction, this occurs through contact with a surface that is warmer or colder than the body conducts heat towards or away from an individual. Water conducts heat away twenty five times quicker than air.
An ideal shelter needs to address all these factors to function efficiently. So ideally it needs to provide some insulation to prevent heat loss or gain through radiation, it needs to be wind proof to reduce convection and it needs to offer protection from direct contact with hot or cold surfaces which can conduct heat and this also means being waterproof.
Most people assume that sheltering from cold and wet temperatures is more important than sheltering from the heat, but in desert environments exposure to daytime temperatures exceeding 40 C can kill just as quickly as hypothermia.

Shelter can take infinite forms especially when one considers that your clothing forms part of your shelter system.  At the most basic level in a real survival situation it can be as simplistic as holing down behind a dry stone wall inside a bivi bag, crawling under a broken down vehicle in the desert, digging a simple snow cave, whatever it takes to simply remove yourself from the elements and minimise heat loss or gain.
Shelters can be built completely of natural materials, and indigenous people around the world provide excellent examples of shelters that utilise readily available natural materials to provide the ultimate shelters for the environment they are in. Examples range from the traditional Igloo of the Innuit, the leaf huts of South East Asian tribes, camel and goat wool tents of the Bedouin and Berber, to the grass and mud walled huts of many African tribes.

Knowledge of some simple shelter designs that can be adapted and used in different environments using different materials can be invaluable if you find yourself in a survival situation or if you deliberately intend to travel in remote areas with limited kit.
As well as an understanding of basic shelter designs, experience of the amount of work required to construct a shelter that fulfils it purpose is also useful to appreciate the amount of work that is required.  In northern temperate deciduous woodland, if leaf litter is utilised as the thatching material for a shelter it needs to be over 70cm thick to guarantee being waterproof and to offer sufficient insulation to keep you warm inside, without a sleeping bag. This will take a good 3 hours to build.

One important consideration to take on board when building any shelter is to physically measure that it is long enough to get in, you would be surprised how often people forget to do this realising only once they get in at night that their head is sticking out!
In winter, it may be necessary to incorporate a fire into a shelter design to ensure that a sufficient temperature is maintained within.
In most environments raising yourself of the ground is recommended, either to ensure and insulating layer between you and the ground and/or to reduce the risk of sharing your bed with biting or venomous animals. In its simplest form this can simple by dry leaf litter, ferns, or leafy branches retained by logs (cot wall bed) or if resources and time allow a full raised be can be constructed. If any bed is built it is important when constructing a shelter to take the increased space taken up by a bed into account.
Whatever shelter you opt for, plan carefully before deciding on the final location.  Chose areas that are naturally sheltered, avoid open and exposed areas.  Pick an area where the resources are to hand, both construction materials for the shelter as well as firewood, water, food etc. Avoid low lying areas, the bottom of steep slopes, dried up river beds or flood plains that are prone to flooding. Valley bottoms and dips may also act as cold sinks.  Avoid animal trails, rodent burrows, insect nests, and rocky areas that may be home to snakes and scorpions. Chose flat level areas, there is nothing worth than sleeping with your head downhill! And finally look above for hazards such as dead branches, rock falls etc.