  There are several lines of evidence indicating that the ultra-relativistic
outflows powering gamma-ray bursts (GRBs) are collimated into narrow jets.
However, these are indirect, and the jet structure is rather poorly
constrained. What is more, the jet dynamics have still not been investigated in
detail. It has been suggested that the observed variety between different long
duration events, ranging from bright spectrally hard GRBs, to dimmer and
spectrally softer X-ray flashes (XRFs) may be largely due to different viewing
angles (or lines of sight) relative to rather similar relativistic jets. Here
we describe the current state of knowledge on these topics, explain some of the
most relevant physics behind some of the basic principles, and discuss
prospects for the future.
