Flexible non-linear laser scanning microscope for non-invasive three-dimensional detection, containing at least one radiation source which generates at least one excitation beam of pulsed laser radiation for initiating a secondary radiation emitted by atoms and molecules, flexible transmission optics for transmitting the radiation to a measuring head having focusing optics by which the radiation is focused in a measurement object and the emitted secondary radiation is guided back and directed to at least one detector system, wherein the measuring head is connected to the at least one radiation source (1; 71; 74) so as to be freely pivotable and rotatable in space and flexibly positionable so that upright microscopy, inverted microscopy and microscopy at any solid angles can be carried out, characterized in that - at least one controllable tilt mirror is arranged for deflecting and aligning the at least one excitation beam (11; 12) of pulsed laser radiation of the at least one radiation source in order to align the at least one excitation beam (11; 12) in the measuring head through the transmission optics in such a way that the at least one excitation beam (11; 12) is arranged concentric to an aperture-limited optical element of the measuring head in any position of the measuring head, - a beam splitter for coupling a small portion out of the at least one excitation beam (11; 12) is arranged in the measuring head in front of the focusing optics in order to generate a test beam, - a photodiode is arranged in a plane in the test beam conjugate to a target plane, wherein a centre alignment of the test beam on the photo diode corresponds to the normal alignment of the excitation beam (11; 12) in the target plane, and - position-dependent X and Y signal components (52; 53) are successively generated by deflection of the excitation beam in X and Y direction respectively, wherein the respective maximum (521; 531) of the position-dependent X and Y signal components corresponds to the centre alignment of the test beam on the photodiode, - a control unit is provided for driving the controllable tilt mirror, which control unit, by means of the position-dependent X and Y signal components, aligns the controllable tilt mirror in such a way that the test beam is directed to the centre of the photodiode, so as to achieve a stabilization of direction of the at least one excitation beam irrespective of the magnitude of the position-dependent impairment of the transmission optics. - at least one controllable tilt mirror is arranged for deflecting and aligning the at least one excitation beam (11; 12) of pulsed laser radiation of the at least one radiation source in order to align the at least one excitation beam (11; 12) in the measuring head through the transmission optics in such a way that the at least one excitation beam (11; 12) is arranged concentric to an aperture-limited optical element of the measuring head in any position of the measuring head, - a beam splitter for coupling a small portion out of the at least one excitation beam (11; 12) is arranged in the measuring head in front of the focusing optics in order to generate a test beam, - a photodiode is arranged in a plane in the test beam conjugate to a target plane, wherein a centre alignment of the test beam on the photo diode corresponds to the normal alignment of the excitation beam (11; 12) in the target plane, and - position-dependent X and Y signal components (52; 53) are successively generated by deflection of the excitation beam in X and Y direction respectively, wherein the respective maximum (521; 531) of the position-dependent X and Y signal components corresponds to the centre alignment of the test beam on the photodiode, - a control unit is provided for driving the controllable tilt mirror, which control unit, by means of the position-dependent X and Y signal components, aligns the controllable tilt mirror in such a way that the test beam is directed to the centre of the photodiode, so as to achieve a stabilization of direction of the at least one excitation beam irrespective of the magnitude of the position-dependent impairment of the transmission optics.