Progress in photodynamic therapy (PDT) depends on the development of: (1) photosensitizers, (2) optical devices among which are lasers and light delivery systems, and (3) clinical procedure. The light delivery systems which are the focus of this article are fiberoptic devices developed in Lausanne, Switzerland for use in the endoscopic treatment of cancer or precancerous lesions in the bronchi, the esophagus, the uterus, the cervix, the upper aerodigestive tract and thoracic cavity. Light delivery systems for both surface and interstitial application are presented, together with some of the physical principles on which they are based. Incorporation in these devices of the possibility for in-situ measurement of reflected therapeutic light and/or fluorescence emitted by endogenous and/or exogenous dyes allows for improved light and drug dosimetry, as well as the measurement of photobleaching, local oxygenation and other tissue properties. The necessity of information on tissue optical parameters, as well as the use of simple mathematical models and tissue phantoms, for optimizing light distributing devices is underlined. The devices are optimized for delivering the desired light intensity distribution to the targeted region with minimal losses. In some cases this implies using the device to modify the shape of the hollow organ during PDT, an example of which is given for the case of the esophagus. In another strategy, one adapts the shape of the device to that of the organ, using an elastic balloon catheter. Here examples are given for the uterus, the bronchi and the thoracic cavity. The mechanical properties, the sizes, shapes and materials of the light delivery systems must be optimized for safe use while retaining low cost. Furthermore, the devices must whenever possible be rendered compatible with existing medical technology. A significant improvement in clinical efficacy has been demonstrated in the testing of some of these new fiberoptic light delivery systems. For endoscopic PDT in the hollow organs, the design and optimalization of multiple new approaches to light distribution will continue to lead to improved clinical results.