N-body simulations predict that CDM halo-assembly occurs in twophases: a rapid accretion phase dominated by major mergers and a slow accretion phase characterised by a gentle addition of mass to the outer halo. We demonstrate that this two-phase accretion leads to CDM halos of the NFW form. During the fast accretionphase fluctuations in the gravitational potential effectivelyisotropise the velocities of CDM particles and we show that this leadsto an inner profile ρ(r) ∝ r-1. Slow accretion onto anestablished potential well leads to an outer profile with ρ(r) ∝ r-3. Using a simple analytic model thatcaptures much of the important physics we show thatthe inner r-1 profile of CDM halos is a natural result of hierarchical mass assembly with a initial phase of rapid accretion.