Hi Linda: This is a topic that comes up every so often. Suggest you go to the Library and look up the Aug issue of Fine Home Building, which has a great article on how to wet proof your basement. (I think it is Aug....could be July)
In summary, the article covers the following for existing homes: - The most common cause of a leaking basement are gutters overflowing or downspouts not moving the water away from the foundation. - The second most common cause of a wet basement is the grading along the sides of the house is not pitched away from the house, thus allowing water to collect along the basement walls where it runs down to the footing, finding all cracks along the way.
These two causes account for about 90% of leaking basements. The article goes on to describe other less common causes, such as a high water table, soil composition and water stauration, etc.