The general characteristics of the emission structures observed in HH jets can be modeled as the result of a time-dependent ejection. ``Variable jet'' models have become very popular because simple forms for the proposed variability (e.g., a 2-sinusoidal mode ejection velocity) lead to the formation of emission structures with a striking resemblance to HH jets.
In detail, the agreement between variable jet model predictions and observations is not always so good. In order to model different objects, several possibilities have been studied: e. g. multi-mode and ``random'' variabilities, jets with non-top hat cross sections, presence of magnetic fields, and precession, orbital and translational motion of the source. This talk will discuss some of this work, and its success at reproducing the characteristics of HH jets.
Finally, recent observations and models of the proper motions of the knots along some HH jets (in particular, HH 1/2, HH 34 and HH 46/47) will be presented.