The realization that electron localization in disordered systems (Anderson localization) [1] is ultimately a wave phenomenon [2,3] has led to the suggestion that photons could be similarly localized by disorder [3]. This conjecture attracted wide interest because the differences between photons and electrons -in their interactions, spin statistics, and methods of injection and detection -may open a new realm of optical and microwave phenomena, and allow a detailed study of the Anderson localization transition undisturbed by the Coulomb interaction. To date, claims of threedimensional photon localization have been based on observations of the exponential decay of the electromagnetic wave [4,5,6,7,8] as it propagates through the disordered medium. But these reports have come under close scrutiny because of the possibility that the decay observed may be due to residual absorption [9,10,11], and because absorption itself may suppress localization [3]. Here we show that the extent of photon localization can be determined by a different approach -measurement of the relative size of fluctuations of certain transmission quantities. The variance of relative fluctuations accurately reflects the extent of localization, even in the presence of absorption. Using this approach, we demonstrate photon localization in both weakly and strongly scattering quasi-one-dimensional dielectric samples and in periodic metallic wire meshes containing metallic scatterers, while ruling it out in three-dimensional mixtures of aluminum spheres.In the absence of inelastic and phase-breaking processes, the ensemble average of the dimensionless conductance g ≡ G /(e 2 /h) is the universal scaling parameter [12] of the electron localization transition [1]. Here ... represents the average over an ensemble of random sample configurations, G is the electronic conductance, e is the electron charge, and h is Planck's constant. The dimensionless conductance g can be defined for classical waves as the transmittance, that is, the sum over transmission coefficients connecting all input modes a and output modes b, g ≡ Σ ab T ab (Ref. 13). In the absence of absorption, g not only determines the scaling of transmission quantities, such as T ab and T a = Σ b T ab that we will refer to as the intensity and total transmission, respectively, but it also determines their full distributions [14,15,16]. In electronically conducting samples or in white paints, g ≫ 1 and Ohm's law holds, g = N ℓ/L, where N is the number of transverse modes at a given frequency, ℓ is the transport mean free path, and L is the sample length. But beyond the localization threshold, at g ≈ 1 (Refs. 12,17), the wavefunction or classical field is exponentially small at the boundary and g falls exponentially with L. Localization can be achieved in a strongly scattering three-dimensional sample with a sufficiently small value of ℓ (Ref. 2), or even in weakly scattering samples in a quasi-one-dimensional geometry of fixed N , once L becomes greater than the localization length, ξ = N ℓ (Ref. 1...