The bright, broad-band afterglows of Gamma-Ray-Bursts (GRBs) can be used to probe the ionization and metal enrichment histories of the intervening intergalactic medium during the epoch of reionization. In contrast to other sources, such as galaxies or quasars, which fade rapidly with increasing redshift, the observed infrared flux from a GRB afterglow at a fixed observed age is only a weak function of its redshift. GRB afterglows may also be microlensed by intervening stars or MACHOs. Due to the superluminal expansion of GRB sources on the sky (at a speed 10-100c), a typical microlensing event at a cosmological distance only lasts for hours to days. GRB000301C shows an unusual achromatic, short time-scale variability, which can be well fitted by a microlensing model.