I once heard something like "inner forms of reductive groups have the same representation theory". 


Is this assertion misguided? 


If this assertion is not misguided, then is there a precise statement to this effect (perhaps in Tannakian terms)? 



Representation theory of inner forms
I vote for "misguided". The representation theory of inner forms are certainly not the "same". What is true (over a local field) is they have the same L-group. A precise version is: the L-packets for G embed in the L-packets for the quasisplit form (assuming the local Langlands conjectures of course).


