The diameter of a graph G is the maximal distance between two vertices of G. A graph G is said to be diameter-edge-invariant, if d(G−e) = d(G) for all its edges, diametervertex-invariant, if d(G − v) = d(G) for all its vertices and diameter-adding-invariant if d(G + e) = d(e) for all edges of the complement of the edge set of G. This paper describes some properties of such graphs and gives several existence results and bounds for parameters of diameter-invariant graphs.