Dow Theory is a particular form of technical analysis based on a presumed linkage btwn 2 well known and followed indices, the Dow Industrials and the Dow Transports.  not everyone agrees that it is useful, as in all techniques, but i personally use it along with others.  i think it is important since it is so well known and followed.  if the Transports aren't following a rally in the Industrials that shows that there is a non-confirmation or divergence warning of underlying, underappreciated problems in the real economy.  can't have true industrial production if the transportation companies aren't moving the resources.  it also fits nicely if you believe that there is a PPT that manipulates the small set of companies that comprise the Dow Industrials upwards on a regular basis w/o having to pump money into all stocks across the board.  lotsa ppl focus on the Dow Industrials.  i think this is true.  everyone i know, from taxi cab drivers, to sophisticated money managers follow the Industrials on their phones for signs of trouble in the economy.  yeah, the SPX would probably be a better indicator but like i said, manipulators should theoretically manipulate the easiest manipulable indices possible and that would be the Industrials.  anyways here is more info:

https://en.wikipedia.org/wiki/Dow_theory