It is also possible the foundation was not properly waterproofed when constructued.
The most difficult water problems are those caused by building in a high ground water table or to close to a natural spring.
Fixing water penetration of the foundation:
A seal on the outside of the foundation wall is best. This prevents water saturation of the wall itself and the underground water pressure presses the sealing material to the foundation tightening the bond. An inside water seal allows wall saturation and the hydrostatic pressure tends to push the interior sealant system away from the wall, increasing the likelyhood of sealant failure.
The best solution may be to excavate and seal the exterior however this is often cost prohibitive and damaging to surrounding landscape an other exterior obstacles.
An interior sealant and treatment of any cracks or perforations of the foundation could be a lower cost alternative to external excavation.
Depending on the severity of the water problem, the use of the basement space and its layout, a drain/pumping system may be a solution. If the water penetration is in predominately one location or on one wall the water may be directed to a drain in the floor connected to a dry well. If a drain is not possible, a small basin may be opened in the floor where water is accumulated and the removed automatically by a sump pump.
If the water can be directed along a wall or walls to a drain or sump pump the remainder of the basement may be recovered for some "dry" uses. The area of drain and pump may be partioned off to separate it from the remainder of the basement. If so, a dehumidifier should be installed in the partioned space to help combat mold and mildew. The space should remain accessible and inspected periodically.