The distinguishing index D′(G) of a graph G is the least number of colours in an edge colouring that breaks all nontrivial automorphisms of G. This invariant is defined only for graphs with at most one isolated vertex and without K2 as a component (we call them admissible graphs). The general upper bound for connected admissible graphs is D′(G)≤Δ(G) except for small cycles C3,C4,C5. Moreover, it was proved by the second author that the equality holds only for cycles of length at least six, symmetric and bisymmetric trees and for two small graphs K4,K3,3. Recently, we proved with Imrich and Woźniak that D′(G)≤⌈Δ(G)√⌉+1 for every connected graph of order at least seven and without pendant edges. There are classes of graphs with the distinguishing index at most two (perhaps with few known exceptions). For instance, traceable graphs, claw-free graphs, Cartesian powers of connected graphs, and 3-connected planar graphs (the latter result was recently proved by Pilśniak and Tucker). A number of open problems will be formulated.