A Direct Numerical Simulation of a particle-laden spatially developing turbulent boundary layer up to Re_θ = 2500 has been performed. The peculiar feature of a boundary layer flow seeded with heavy particles is the variation of the local dimensionless parameters defining the fluid-particle interactions along the streamwise direction. An inner and an outer Stokes number can be defined using inner and outer flow units. These two parameters show different decay rates in the streamwise direction so that it is possible to find a decoupled particle dynamics between the inner and the outer region of the boundary layer. Preferential near-wall particle accumulation is similar to that observed in turbulent channel flow, while a self-similar behavior characterizes the outer region.