  The advent of ultrafast pulsed X-ray free-electron lasers with very high
brightness has enabled the determination of transient molecular structures of
small and medium-sized organic molecules in excited states and undergoing
chemical dynamics using X-ray scattering. This chapter provides an introduction
into X-ray scattering theory and considers several important aspects relating
to the experimental implementation. Ultrafast gas phase X-ray scattering is
shown to provide new observables to elucidate the dynamics of chemical
reactions by providing complete, time-dependent molecular structures.
Consideration of correlations between structural parameters are important for
molecules far from their equilibrium and the changes in electron density
distributions of molecules upon optical excitation need to be considered in the
analysis. Future technological developments are expected to lead to further
important advances.
