  The next generation of weak gravitational lensing surveys is capable of
generating good measurements of cosmological parameters, provided that, amongst
other requirements, adequate redshift information is available for the
background galaxies that are measured. It is frequently assumed that
photometric redshift techniques provide the means to achieve this. Here we
compare Bayesian and frequentist approaches to photometric redshift estimation,
particularly at faint magnitudes. We identify and discuss the biases that are
inherent in the various methods, and describe an optimum Bayesian method for
extracting redshift distributions from photometric data.
