How to see the forest for the trees

One of the major starting points of discrete optimization is the theorem of Nash-Williams and Tutte on the existence of $k$ disjoint spanning trees of a graph along with its counterpart on the existence of $k$ forests covering all edges of the graph. These elegant results triggered a comprehensive research that gave rise to far-reaching generalizations and found applications at seemingly far-fetched areas. There are well over a thousand papers in the literature, including quite a few brand-new ones. Our first goal is to enlighten some aspects and links of these developments with the hope that the melody finds its way to non-experts. But we hope that experts will also find some novelties in our orchestration.