The ultra-high sensitivity of the 1S0 nucleon-nucleon (NN) scattering length to the strength of the nuclear potential makes it a central parameter for quantifying charge symmetry breaking in the strong nuclear force. Of the three 1S0 NN scattering lengths, the value of the neutron-neutron (nn) scattering length ann is the most uncertain. A number of reactions that produce two neutrons with low relative momentum have been used to determine ann. However, the two reactions that give ann with the least theoretical uncertainty are pion-deuteron capture (π-+d → n+n+γ) and neutron-deuteron breakup (n+d → n+n+p). Discrepancies between the results of different nd breakup experiments raise serious concerns about the use of this reaction to determine ann. In this paper the results obtained using these two popular reactions will be summarized and discussed in the context of the experimental techniques and theory used to determine ann in selected experiments.