Trees. Both beautiful and useful. But we’re not talking about the green, oxygen-providing ones. As abstract structures we see trees all over the place — file systems, class hierarchies, call trees, ordered data structures, etc. They are neat and tidy, nested and hierarchical — a simple way of organising things; a simple way of breaking large things down into small things. The problem is, though, that there are many things — from modest fragments of code up to enterprise-wide IT systems — that do not fit comfortably into this way of looking at the world and organising it. Software architecture, design patterns, class decomposition, performance, unit tests... all of these cut across the strict hierarchy of trees. This talk will look at what this means for how we think and design systems, whether large or small.