Actinic
Actinic keratosis (also called solar keratosis, or AK) is a premalignant condition of thick, scaly, or crusty patches of skin. It is  most common in fair-skinned people who are frequently exposed to the sun, because their pigment isn't very protective. It usually is accompanied by solar damage. Since some of these pre-cancers progress to squamous cell carcinoma, they should be treated.
When skin is exposed to the sun constantly, thick, scaly, or crusty bumps appear. The scaly or crusty part of the bump is dry and rough. The growths start out as flat scaly areas, and later grow into a tough, wart-like area.
An actinic keratosis site commonly ranges between 2 and 6 millimeters in size, and can be dark or light, tan, pink, red, a combination of all these, or have the same pigment as the surrounding skin. It may appear on any sun-exposed area, such as the face, ears, neck, scalp, chest, backs of hands, forearms, or lips.

# Prevention
Preventive measures recommended for AK are similar to those for skin cancer:
- Not staying in the sun for long periods of time without protection (e.g.:sunscreen, clothing, hats).
- Frequently applying powerful sunscreens with SPF ratings greater than 15 and that also block both UVA and UVB light.
- Using sunscreen even in winter sun exposure.
- Wearing sun protective clothing such as hats, long-sleeved shirts, long skirts, or pants.
- Avoiding sun exposure during noon hours is very helpful because ultraviolet light is the most powerful at that time.

# Diagnosis
Doctors can usually identify AK by doing a thorough examination. A biopsy may be necessary when the keratosis is large and/or thick, to make sure that the bump is a keratosis and not a skin cancer. Seborrheic keratoses are other bumps that appear in groups like the actinic keratosis but are not caused by sun exposure, and are not related to skin cancers. Seborrheic keratoses may be mistaken for an actinic keratosis.

# Treatment
Various modalities are employed in the treatment of actinic keratosis:
- Cryosurgery, e.g. with liquid nitrogen, by "freezing off" the AKs.
- 5-fluorouracil (a chemotherapy agent): a cream that contains this medication causes AKs to become red and inflamed before they fall off.
- Photodynamic therapy: this new therapy involves injecting a chemical into the bloodstream, which makes AKs more sensitive to any form of light.
- Laser , notably CO2 and Er:YAG lasers. A Laser resurfacing technique is often used with diffuse AKs.
- Electrocautery: burning off AKs with electricity.
- Immunotherapy: topical treatment with imiquimod (Aldara), an immune enhancing agent
- Different forms of surgery.
- Crocodile oil: is available as a skin balm and has been used successfully as is a natural remedy.
Regular follow-up after treatment is advised by many doctors. The regular checks are to make sure new bumps have not developed and that old ones haven't become thicker and/or have skin disease.

# Experimental treatments
In 2007, Australia biopharmaceutical company Clinuvel Pharmaceuticals Limited began clinical trials with a melanocyte-stimulating hormone called melanotan which they refer to with the proprietary name CUV1647 for actinic keratosis in organ transplant patients.
Another Australian biopharmaceutical company Peplin  is also developing a topical treatment for actinic keratosis. Formed in 1998 they are currently developing PEP005, which is the first in a new class of compounds and which is derived from Euphorbia peplus, or E. peplus, a rapidly growing, readily-available plant, commonly referred to as petty spurge or radium weed. The sap of E. peplus has a long history of traditional use for a variety of conditions, including the topical self-treatment of various skin disorders, such as skin cancer and pre-cancerous skin lesions. The company has recently redomiciled to the USA and is about to enter phase III trials with PEP005.