The present invention relates generally to catalysts and, in particular, relates to in-situ replacement of catalyst within a reactor. In an embodiment, the present invention comprises a method for the in-situ replacement of catalyst bodies in a catalytic reactor comprising removing catalyst bodies in-situ from at least one modularized section of a catalytic layer within a catalytic reactor and replacing the removed catalyst bodies in-situ with replacement bodies comprising catalytic function wherein at least 10% of the total catalyst bodies within the layer are replaced.