Depending on your environment, shelter can be the most important item for survival. Extreme conditions can kill a person in as little as three hours. Humans can't survive long in most environments without some kind of shelter to protect them from the elements. Shelter offers protection from extremes of heat, cold, intense sun, and prolonged precipitation. Clothes are our "micro" shelter which allow us to operate in extreme environments enabling us to gather our survival needs such as water and food. Eskimos survive where they live because their clothes are their shelter. For most of us clothes are a temporary substitute for a more efficient shelter which better controls our environment. The ability to control our environment with shelter has allowed us to live in areas we simply couldn't survive other wise. Most of us these days live in wood frame construction homes which are generally adequate for any climate. With the addition of better insulation and energy efficient environmental control units, we are better off than most were 50 years ago. We've come a long way from living in caves! Your primary shelter should be your main concern and focus for every day survival. You should, however, consider acquiring or learning to construct alternate shelter options. If your primary shelter becomes unlivable, you will need an alternative. Simply having a tent or tents to accommodate everyone in your home is a good first step. They are easy to assemble and will provide some protection from the environment. Learn to construct simple shelters from materials readily available where you live. Be ready for any local scenario for any season. You may not have time to build an elaborate shelter so be ready ahead of time and practice your skills in construction. Just getting out of the wind and rain could be the difference between life and death. The most simple shelters such as, lean-to's and A-frames can be constructed from tarps and para-cord or clothes line. Insulation with twigs and leaves increase the effectiveness of your control over the environment. The types of shelters you should learn depends on what climate you live in. Temperate regions require you to learn many types of shelters as the seasons will be radically different throughout the year.  If you live in a tropical environment there will be no need to learn how to build a snow cave or igloo. Likewise if you live in the arctic you are going to want a shelter that has a heat source. Basic shelters in no particular order include: round lodge, tipi, wicki-up, ramada, quinzhee, igloo, snow cave, wedge tarp, tarp wing, tarp burrito, tarp tipi, A-frame,  desert tarp, tarp hammock, bough bed, leaf hut, lean-to, debris hut, wigwam, pit shelter, yurt, tent, etc...