In this talk, I will present our recent theory on the self-sustained current oscillations (SSCO) in spin-blockaded double quantum dots. Our theory is based on the experimental fact that the SSCO in spin-blockaded double quantum dots is closely associated with the dynamically polarized nuclear spins. We show that dynamically polarized nuclear spins undergo a sequence of semistable limit cycle, Hopf, and homoclinic bifurcations occur as an external field varies. Although the fundamental time scale is nanoseconds for electron tunneling and micro seconds for nuclear spin dynamics under an external or Overhauser field of order of Tesla, the divergent period near the homoclinic bifurcation explains well why the period in experiments can be many orders of magnitude longer than all microscopic time scales. Some predictions associate with the theory may also be tested experimentally.