With the development of new photosensitizers that have enhanced photoactivation at longer wavelengths than haematoporphyrin derivative, new considerations arise in the light source and delivery systems and in the techniques for physical dosimetry and in vivo optical measurements in photodynamic therapy. The limitations and future potential of solid-state laser sources are presented. The relationships between photosensitizer photoactivation characteristics and the effective photodynamic treatment volume are developed and discussed quantitatively. The problems in defining and measuring the photodynamic dose are examined, and potential techniques for measuring the factors involved in this are evaluated with emphasis on noninvasive approaches which may be used clinically.