ALA-induced protoporphyrin IX (PpIX) is used for fluorescence diagnosis (ALA-FD) 
and for fluorescence-guided resection of both (pre)malignant and non-malignant 
diseases. ALA is also applied in photodynamic therapy (ALA-PDT) of superficial 
(pre)malignant lesions in dermatology, urology, neurosurgery, 
otorhinolaryngology, gynecology and gastroenterology. Today, ALA is approved as 
Levulan for actinic keratoses, the ALA-methyl ester Metvix for actinic keratoses 
and basal cell carcinoma, the ALA-hexyl ester Hexvix for the diagnosis of 
bladder cancer and Gliolan for malignant glioma. The use of ALA for PDT and FD 
was established around 25 years ago, with most of the fundamental knowledge 
gained at the "bench" and implemented at the "bedside" due to the diligence of a 
few researchers within the first 10 years of research. After 1993 ALA research 
was taken up by many groups. For patient treatment, several factors are 
relevant. Administered mainly in a topical or oral form, ALA penetrates tissue 
in a sub-optimal way, which is currently improved by special techniques and the 
use of ALA-esters. PpIX accumulation is elevated in many malignant tissues, 
several tissue abnormalities, and in mucosa. It is also found at elevated levels 
in macrophages, dendritic cells and activated lymphocytes. Following sufficient 
PpIX accumulation in the target cells, irradiation is carried out which may be 
accompanied by a burning sensation at the treatment site. Due to a saturation 
process of PpIX formation and rapid photobleaching during irradiation the risk 
of overtreatment is relatively low. Pharmacokinetical studies have demonstrated 
a low systemic photosensitivity and excretion of PpIX via natural routes.