How many elementary particles are there? I was reading QED by Feynman (a dated book) and he seemed to be suggesting we have discovered hundreds of particles. But I thought there were only the ones we saw in the standard model (ie leptons, quarks, gauge bosons, higgs boson)? Why aren't all those hundreds of particles included in the typical "periodic table" for elementary particles?
 A: After  the quark model was established and the standard model was accepted as the theoretical framework for elementary particles, it became obvious that the great number of resonances could be organized into group representations of the standard model's group structures. 
These are still recorded and sought after according to the predictions of the standard model. The particle data group does a good job of keeping the records.
A: Part of the problem lies with the definition of "elementary". Before the discovery of nuclear physics atoms were considered elementary. This chemical definition overlooked the (then unknown) fact that most naturally occurring elements are mixtures of different isotopes. After nuclear physics was established, the electron, proton and later neutron were considered elementary, although that didn't last long. Soon it became clear that protons and neutrons themselves are made of constituents. Now we consider quarks elementary. This, of course, is a very good definition up to the TeV energy range because the current experiments show no sign of there being an internal quark structure (and much less so an internal structure for electrons). 
While some are sufficiently confident that we have peeled back the last layer of this onion, it's a good idea to keep in mind that our predecessors thought the same thing after they had discovered atoms etc.. On some level, of course, we have become more cautious with the use of the word, as string theory shows, which tries to reduce the current zoo of "elementary" particles to an even smaller one, in which everything can be described as vibrations of strings. 
I think it's a pretty good bet that today's "elementary particles" will be modeled as excitations of something even more fundamental tomorrow. 
