  We present a review of the present state of knowledge regarding the relative
ages of Galactic globular clusters. First, we discuss the relevant galaxy
formation models and describe the detailed predictions they make with respect
to the formation timescale and chemical evolution of the globular clusters.
Next, the techniques used to estimate globular cluster ages are described and
evaluated with particular emphasis on the advantages and disadvantages of each
method. With these techniques as a foundation, we present arguments in favor of
the following assertions: 1) The age of a globular cluster is the likeliest
candidate to be the global second parameter, which along with metal abundance,
controls the morphology of the horizonal branch. 2) A total age range of as
much as $\sim$5 Gyr exists among the bulk of the Galactic globulars. 3) There
is a significant relation between age and metallicity among the Galactic
globular clusters if the slope of the \mvrr-\feh relation is less than
$\sim$0.23. These conclusions along with other supporting evidence favor a
formation scenario in which the inner regions of the Galactic halo collapsed in
a monotonic fashion over a short time period much less than 1 Gyr. In contrast,
the outer regions of the halo fragmented and collapsed in a chaotic manner over
several Gyrs.
