In this paper we describe a photon migration approach for modeling fluorescence in an optically thick, turbid medium such as human tissue. In such a medium the intrinsic fluorescence spectrum of the fluorophores Φ can be distorted by the interplay of many factors, including scattering and absorption, excitation and collection geometries, and boundary conditions. The model provides an analytical relationship between the bulk fluorescence spectrum F and the diffuse reflectance spectrum R for arbitrary geometries and boundary conditions. We demonstrate that the distortion can be simply and accurately removed by measuring R from the optically thick medium over the same wavelength range and in the same manner as F. Over a wide range of tissue parameters this relationship may be written as Φα F/R(eff), with R(eff) a corrected form of the measured diffuse reflectance. The validity of this approach is demonstrated in both laboratory experiments on human aortic media and by comparison with Monte Carlo simulations and dif usion theory. Connection with a previous algorithm for extracting intrinsic fluorescence is also discussed.