Forest canopies are highly diverse components of forest ecosystems, yet they remain underexplored in many regions of the world. One example is the Nothofagus rainforests of northern Patagonia. In this chapter, we summarize findings from our canopy explorations in west Andean forests between 39°S and 40°S. Old-growth forests in this region are characterized by an abundance of large, old trees, some exceeding 3 m in diameter at breast height (DBH) and reaching heights of over 40 m. These canopies harbor high biodiversity, much of it concentrated in large old trees. Their crowns accumulate substantial arboreal soil biomass and standing dead branches, which we refer to as canopy coarse woody debris, both of which host rich communities of plants, fungi, vertebrates, and invertebrates. The functional consequences of this canopy biodiversity are potentially profound. Epiphytes can increase a tree’s total foliage biomass by 50–100%. The abundance of dead organic material supports detritus-based food webs that may sustain the high densities of invertebrates and vertebrates we have documented. These canopies also host ecological interactions that are characteristic of old-growth forests, such as ecosystem engineering by Fascicularia bicolor, hemi-epiphytism by Raukaua laetevirens, and advanced canopy regeneration, i.e., the establishment of tree seedlings and saplings in the canopy that eventually colonizing treefall gaps when host trees collapse. Large old trees provide two fundamental structural components, the arboreal soils and canopy coarse woody debris, that support the species and processes described above. The preservation of large old trees is essential to maintaining biodiversity. Our research underscores that the biodiversity and ecological processes that develop on trees over centuries require increased conservation attention.