Not being able to mutate elements through non-const iterators is surprising enough that I forget about it every time I use Boost.MultiIndex. Part of the reason for this is that this information is buried under section "multi_index/doc/tutorial/basics.html#seq_indices". I suggest that the following paragraph: "That is, iterators of a sequenced index (of all types of indices, actually) point to constant elements. This limitation might come as a surprise, but it is imposed by the way multi_index_containers work; if elements were allowed to be changed in this manner, we could introduce inconsistencies in other ordered indices of the multi_index_container. Element modification can nevertheless be done by means of update operations." be moved under "multi_index/doc/tutorial/basics.html#iterator_access" as a call-out.