A method of navigation in three-dimensional, electronic image data sets is disclosed, whereby the image data sets contain three-dimensional image data subsets. The method includes optically representing at least one two-dimensional projection of an image data set that includes a two-dimensional subprojection of at least one image data subset, optically highlighting the at least one two-dimensional subprojection, receiving a user input directed to the selection of a certain subprojection, and optically representing, in accordance with said user input, at least one additional two-dimensional projection of the image data set that includes a two-dimensional projection of the selected image data subset. The method can be used for an image data set that is formed by fusion of at least two source image data sets. The source image data sets can especially comprise computer tomography and a positron-emission tomography image data set.