When to use CP or ND filter

Im looking to try my hand at landscape photography and i just wanted to know when do you know when to use a CP filter or a ND filter?
thanks
Steve

The primary use of a circular polarizer is to control reflections, whether they are from water, foliage, or particles in the sky. The primary use of an ND filter is to reduce the amount of light reaching the camera's sensor, which can enable slower shutter speeds to create motion blur (in waterfalls, for example). Some ND Filters are graduated (that is, their effects fade from one end of the filter to the other); these filters can help to balance scenes that have a lot of light in only one area (for example, a bright sky versus darker land). These latter types of filters (called "GNDs" for short) are going out of fashion in the digital era because digital editing techniques have largely rendered them obsolete.

It isn't always obvious when a CP is going to be useful. You'll hear all sorts of nonsense about using them, but really the only way to know for sure is to look though one and rotate it to see if it helps the scene. Remember you don't have to use it at maximum polarization.

During the day I usually wear polarized sunglasses and people must think I'm mad as I twist my head from side to side to see if I might want to use a CP.

Dusk/dawn can be more tricky. Since I'm not usually worried about long shutter speeds, I'll often put one on even if I don't think it will be useful.

Ok thanks guys

Careful if you use one on a wide angle lens. If you get a lot of sky in your photo, the polarization will be uneven, and that will mean an uneven sky, that is, dark blue in one section, lighter blue in another.

(of course, the reason I know this is I took a boatload of photos with a wide angle lens, then noticed the uneven blue sky only after I uploaded them to my computer)

Patrick thanks for the tip