A recent reformulation of the laser field equations using the true modes of the laser cavity and including a noise polarization term to account for spontaneous emission shows that conventional understanding of amplifier noise figure oscillator linewidth and oscillator build-up time needs to be modified for laser amplifiers or oscillators with nonorthogonal transverse eigenmodes. Gain-guided lasers and unstable-resonator lasers in particular have nonorthogonal transverse modes and hence exhibit unconventional noise behavior. One important consequence of this theory is the appearance of an excess spontaneous emission factor (the " Petermann factor" ) which multiplies the well-known Schawlow-Townes formula for the laser linewidth. This factor can be substantially greater than unity for unstable resonators with large magnification or Fresnel number. We discuss our investigations of the consequences of the nonorthogonality of the transverse eigenmodes especially on laser linewidth in unstable-resonator lasers. We believe that the results of these investigations are important where the temporal coherence properties of the laser are of interest namely in coherent optical communications spectroscopy optical wavelength standards and also in the injection seeding of pulsed high-power unstable resonator lasers. Extensive calculations of the excess noise factor in real hard-edged unstable resonators have been carried out using a virtual-source approach to calculate the exact unstable-resonator eigenmodes. An experiment is also being carried out to demonstrate the linewidth enhancement in a small diode-pumped geometrically unstable laser oscillator.