My basement is very moist and has a musky smell . Any ideas ?

1. Check foundation and walls for cracks, or seepage around windows and doors and get them repaired.
2. Use a dehumidifier.
3. Remove old carpeting and draperies.
4. Get boxes and bins off the floor to allow for proper air circulation. Put new boxes (throw old ones out) on blocks or shelves.
5. Declutter.
6. As long as there is no flooding, use a subfloor that allows air circulation, there are different brands: http://barricadesubfloor.com/
7. Use a mould fogger like this one:

http://www.concrobium.com/en-can/products/mold-control-fogger/

8. If your walls are drywalled, repaint with a mould and mildew blocking primer and paint (like the ones used in bathrooms).

Definitely use a dehumidifier. We purchased one for our basement and the whole house smells better.

Poor ventilation, and a breeding ground for mold. Get dehumidifiers and use fans to improve air circulation. If any windows, open them up!

The replies that are posted so far are excellent! I'm only going to add a few things.

There are things in basements that hold the moldy smell like particle board and paper. A dilute bleach solution can kill quite a bit of the mildew smell. Mildew=water and lack of circulation, as others have stated.

Be sure your gutters are directed away from your home and that the soil is graded to direct water away from your basement.

You're welcome, good luck!