  Understanding the bimodal duration distribution (dynamic range $>$ 10$^{4}$)
of $\gamma$-ray bursts is central to determining if the phenomenon is in fact a
singular one. A unifying concept, beyond isotropy and inhomogeneity of the two
groups separately, is that bursts consist of pulses, organized in time and
energy: wider pulses are more asymmetric, their centroids are shifted to later
times at lower energies, and shorter, more symmetric pulses tend to be
spectrally harder. Long bursts tend to have many pulses while short bursts
usually have few, relatively narrow pulses. Two factors, viewing angle and
beaming, may account for pulse asymmetry and the large dynamic range ($\sim$
200) in pulse widths. A cosmological time-dilation signature, with an expected
dynamic range of order two, would be difficult to measure against these large
intrinsic variations and low signal-to-noise levels of dimmer bursts. Some
statistics ($T_{\rm 90}$, pulse intervals) are particularly sensitive to
brightness bias, noise, and apparently minor variations in definition. Also,
spectral redshift would move narrower, high-energy emission from dim bursts
into the band of observation, constituting a countering effect to time
dilation. With analyses restricted to bursts longer than $\sim$ 2 s, tests for
time dilation that are constructed to be free of brightness bias have yielded
time-dilation factors $\sim$ 2--3, for pulse structures, intervals between
structures, and durations.
