There are a number of potential causes for ridges on fingernails. Vertical ridges which run the length of the nail are actually quite common, especially among older people, although they can be a cause for concern, especially when paired with color changes in the nail. Horizontal ridges are often an indicator of a health problem which requires medical attention. If you have just noticed ridges in your nails, you shouldn't panic, but you may want to make time to see a doctor to discuss the change.
One extremely common cause of ridges on fingernails is malnutrition or malabsorption. Many vitamin deficiencies cause ridges on fingernails, as the body's production of the materials in the nail is interrupted. Eating a healthier diet and supplementing with vitamins can help clear up the ridges; for especially strong, health nails you can eat gelatin, which will also benefit your hair. If improvements to your diet do not resolve the ridges on your fingernails, you may have a disorder which interrupts the absorption of nutrients by your body, and you should see a doctor.
Another cause of ridges on fingernails is lack of moisture. If your nails are ridged or cracked, massaging the nails or cuticles with a moisturizing cream or oil can be very beneficial, as can increasing the amount of water you drink. If you live in a hot climate or do a lot of manual labor, your hands can tend to dry out, and in addition to keeping your nails healthier and happier, regular moisturizing will also benefit the skin of your hands.
Changes in the nail can also be caused by disease. Dramatic changes in nail structure can indicate respiratory or circulatory problems, along with autoimmune disease, and in some cases nails change color or develop ridges when the body is exposed to a toxin such as arsenic. Some skin conditions like psoriasis are also linked with changes like ridges on fingernails, and pitting of the nail.
Changes in nail health can be early indicators of changes in the general health of your body. For this reason, it is a good idea to pay attention to the state of your nails. While your nails can be ridged and perfectly healthy, deep ridges can be a sign of a problem, and horizontal ridges are especially dangerous. Likewise with dramatic color changes in your nails. If your nails turn blueish, greenish, or yellowish, it is a sign that your body is having difficulty coping with a problem, and you should see a doctor.