A nonlinear laser scanning microscope for flexible, noninvasive three-dimensional detection comprising a measuring head which is flexibly connected to at least one radiation source by transmission optics and can be freely positioned in space, at least one controllable tilt mirror is arranged for aligning the excitation beam in order to keep the excitation beam concentric to an aperture-limited optical element of the measuring head, a test beam which is coupled out of the excitation beam onto a spatially resolving photodetector for monitoring the center alignment of the test beam as a conjugate position to the target position of the excitation beam and directional stabilizing the excitation beam by a control unit of the tilt mirror depending on a determined deviation.
