"Models are fine as long as you don't believe in them" 
Anonymous field geologist
Constraints provided by observations of terrestrial impacts
•Cross-sectional information both through geophysics and observations of craters at different erosional levels.
1.Not much disruption of strata outside of crater.
2.Central structures are uplifted deep strata.
•Compositional information at a variety of scales.
1.Overturned flap model, surge emplacement of ejecta.
2. Mixing of impactor with target rocks.
•Information on volume of melt, degree of disruption, shockwave pattern and pressures, constrain "contact and compression stage".
•A major potential problem is that only a handful of "type examples" have been studied at a level beyond identifying the presence of shock features and then age dating.  Each one of these type examples seems significantly different from the others, which calls into question which observations can be considered generally applicable to all craters.
•Another potential problem – craters are so complicated that it is very difficult to detach observation from interpretation.

Constraints provided by observations of planetary craters
•Craters, particularly their internal features, appear generally the same on all the planets.  While onset of different features (e.g., terraces, central peaks) varies from planet to planet, the basic features are generally similar.
•Within the range of impactor velocity and composition, final crater form is minimally dependent on impactor properties.  To put this another way, for a given crater diameter on a given planet there are not huge variations in crater appearance that could be attributable to asteroidal versus cometary impactors. 
•Formation of "complex" features all seem to show dependence on target strength and gravity, but it isn't strictly linear and inferred relative strengths cannot be linked to known material properties.
•Target layering can have a major effect on final crater form; this is particularly evident on the icy satellites.
•An atmosphere does not make all the ejecta fall back into the hole.  It does cause the ejecta to be emplaced nonballistically as flows.

Questions for which I would like an answer and/or a research direction identified:
•Are the terrestrial "type examples" really type examples, and if not which other craters we should be studying in great detail?
•Are there types observations of terrestrial and/or planetary craters that we should be making but aren't?
•Is the proportional growth model valid?
•As a follow-up to the previous question, is it a valid idea to think of complex crater formation as starting from a transient cavity?  In other words, are we muddying the waters by envisioning an excavation stage followed by  a modification stage?
•How do central peaks, peak rings, and basin rings form, and are the processes related?
•What properties of the impactor can we determine from the crater, both from a terrestrial crater and from only orbital observations on the planet?