Unintentional but unavoidable fabrication imperfections in state-of-the-art photonic-crystal waveguides lead to the spontaneous formation of Anderson-localized modes thereby limiting slowlight propagation and its potential applications. On the other hand, disorder-induced cavities offer an approach to cavity-quantum electrodynamics and random lasing at the nanoscale. The key statistical parameter governing the disorder effects is the localization length, which together with the waveguide length determines the statistical transport of light through the waveguide. In a disordered photonic-crystal waveguide, the localization length is highly dispersive, and therefore, by controlling the underlying lattice parameters, it is possible to tune the localization of the mode. In the present work, we study the localization length in a disordered photonic-crystal waveguide using numerical simulations. We demonstrate two different localization regimes in the dispersion diagram where the localization length is linked to the density of states and the photon effective mass, respectively. The two different localization regimes are identified in experiments by recording the photoluminescence from quantum dots embedded in photonic-crystal waveguides.In quantum nanophotonics, low-dimensional photonic nanostructures, such as cavities or waveguides, are fabricated in order to enhance the photon-emitter interaction [1]. Importantly, the photon dispersion can be engineered, enabling, e.g., slow-light transport [2] or efficient single-photon sources [3]. However, the optical properties of photonic nanostructures are often rather sensitive to unintentional but unavoidable fabrication imperfections [4,5]. The interplay between order and disorder in a photonic crystal [6,7] or a photonic-crystal waveguide [8-10] may induce strong light confinement due to multiple light scattering. The underlying wave interference process leads to disorder-induced Anderson localization, which was initially developed to explain the metal-insulator phase transition for electron waves [11]. In photonic crystals, the ability to precisely mold the dielectric medium even on a length scale smaller than the photoniccrystal unit cell implies that the photon dispersion relation can be engineered. Consequently, light localization processes can be modified. In the present work, we identify two different regimes of Anderson localization and we extract the governing localization length, ξ. In these two regimes, the localization length is linked to two different underlying properties of the photonic lattice, i.e., either the photonic density of states (DOS) or the photon effective mass.To reveal the two different mechanisms leading to localization, we study the scaling of the localization length, ξ, which is the ensemble-averaged exponential decay of the electromagnetic field intensity. ξ is a key parameter in the localization regime determining the light transport, and therefore is related to the light-matter interaction strength between a quantum emitter and an Anderson...