  Cosmological gamma ray bursts (GRBs) are the brightest explosions in the
Universe. Satellite detectors, such as Beppo-SAX, HETE2 and more recently
Swift, have provided a wealth of data, including the localization and redshifts
of subsets of GRBs. The redshift distribution has been utilized in several
studies in attempts to constrain the evolving star formation rate and to probe
GRB rate evolution in the high-redshift Universe. These studies find that the
GRB luminosity function and/or the rate density evolve with redshift. We
present a short review of the problems of constraining GRB rate evolution in
the context of the complex mix of biases inherent in the redshift measurements.
To disentangle GRB rate evolution from the biases prevalent in the redshift
distribution will require accounting for the incompleteness of the observed
redshift sample. We highlight the importance of formulating a `complete GRB
selection function' to account for the main sources of bias.
