  We present the results of the multi-wavelength campaigns on 3C 273 in
1993-1995. During the observations in late 1993 this quasar showed an increase
of its flux for energies greater equal 100 MeV from about 2.1 x 10^-7
photons/cm^2/s to approximately 5.6 x 10^-7 photons/cm^2/s during a radio
outburst at 14.5, 22 and 37 GHz. However, no one-to-one correlation of the
gamma-ray radiation with any frequency could be found. The photon spectral
index of the high energy spectrum changed from (3.20 +- 0.54) to (2.20 +- 0.22)
in the sense that the spectrum flattened when the gamma-ray flux increased.
Fits of the three most prominent models (synchrotron self-Comptonization,
external inverse Comptonization and the proton initiated cascade model) for the
explanation of the high gamma-ray emission of active galactic nuclei were
performed to the multi-wavelength spectrum of 3C 273. All three models are able
to represent the basic features of the multi-wavelength spectrum. Although
there are some differences the data are still not decisive enough to
discriminate between the models.
