The use of non-invasive and invasive techniques for the assessment of human photodamaged skin is reviewed. Physical changes during photodamage and its treatment are best scored using a visual analogue scale rather than a short, non-equal interval scale. Epidermal thickness can be measured by histometric methods but dermal thickness can be measured non-invasively using pulsed A-scan and B-scan ultrasound techniques. These approaches are not effective in detecting any changes due to photodamage. Mechanical properties of the dermis can be determined using either a static or a dynamic test mode. The authors have used extensometry to provide a measure of the laxity of skin. Replicas of the crow's foot areas have been taken before and after tretinoin treatment, and the replicas have been inspected by optical profilometry. Reductions of blood flow in photodamaged skin have been established using laser Doppler measurements, the effect being reversed by topical tretinoin. Invasive biochemical techniques have the disadvantage that they generally require large amounts of tissue. Cytochemical techniques, however, have shown increased glucose-6-phosphate dehydrogenase activity in the granular cell layer of patients with non-melanoma skin cancer, premalignant epidermal lesions, sun-damaged epidermis and artificially irradiated skin. This technique may provide an important model for the study of photodamage. It is concluded that there is no single method available to quantify the degenerative changes associated with photodamage and the effects of tretinoin.