We investigate the effect of annular apertures in two-photon excitatio
n confocal and non-confocal microscopy. In a theoretical study, we sho
w that the use of annular apertures in two-photon excitation microscop
y leads to resolution increase without diffraction fringes. We determi
ne experimentally the axial resolution of annular aperture confocal an
d non-confocal two-photon excitation microscopes and compare it with t
hat of the standard two-photon excitation contrasts. Images of fluores
cence beads obtained with a transmission annular aperture two-photon e
xcitation microscope demonstrate the applicability of annular aperture
s for enlarging the focal depth in high resolution two-photon excitati
on imaging.