The Role of Dust in GRB Afterglows
  We show that the clumpy structure of star-forming regions can naturally explain the fact that 50-70% of GRB afterglows are optically ``dark.'' We also show that dust echos from the GRB and its afterglow, produced by the clumpy structure of the star-forming region in which the GRB occurs, can lead to temporal variability and peaks in the NIR, optical, and UV lightcurves of GRB afterglows. We note that the detection of GRB ``orphan'' afterglows would provide strong evidence that the star-forming regions in which GRBs occur are clumpy. 
Donald Q. Lamb