A method for monitoring the generation of a three-dimensional object (5) being formed in a tomographic additive manufacturing system from simulated tomographic two-dimensional back-projections (9) of a desired 3D article comprises the steps of: - illuminating a container (3) comprising a photoresponsive material (103) with a light beam (2a) of two-dimensional light patterns generated from said two-dimensional back-projections (9) at multiple angles, preferably by rotation of the container (3), so as to form the object (5) in the container (3); - capturing images of said object (5) being formed by volumetric printing with an imaging system (6, 7, 8) disposed around the container (3) in which the object (5) is being formed; - determining from said images the shape or extent to which the object has been formed.