Herbig-Haro jets record the mass ejection and accretion history of young stars and provide important clues as to how stars form. The use of outflows to constrain the physics of star formation requires an understanding of how shocks within a jet transfer momentum to the ambient medium. Our understanding of how this momentum transfer occurs is improving at a rapid pace driven by (1) spectacular high spatial resolution Hubble Space Telescope images, (2) large format ground-based CCDs with wide fields of view, and (3) velocity-resolved images taken with Fabry-Perot spectrometers and image slicers that enable radial velocities to be measured over a large field of view. HST images of jets resolve the spatial structure of the cooling zones behind the shocks in jets clearly for the first time, and enable us to identify shock fronts and to follow proper motions of subarcsecond structures. Wide field CCDs have shown that outflows from young stars can extend dozens of light years from their sources, which are often multiple systems that drive multiple jets. Velocity and line excitation maps of jets probe the physical conditions within shocked gas, and make possible quantitative comparisons with theoretical models of the flow dynamics. Studies of jets within H II regions are in their infancy, but such objects offer a unique opportunity to observe entire outflows as they are illuminated by ambient ultraviolet light.