The use of engine diagnostic techniques in research on the reciprocating internal combustion engine has contributed substantially to engine progress over the years. Many of these techniques were developed before the advent of the laser, and most engine research still uses these classical methods. This paper provides historical snapshots of efforts to understand flame propagation and knock in homogeneous-charge engines, and fuel-air mixing and some of its ramifications in diesels. Such a review demonstrates the accomplishments facilitated by measurement of pressure, temperature, fluid motions, and chemistry within the cylinder. A critique of these classical diagnostics is then offered.