Conserving the genetic diversity within our tree species and the processes that determine it are important for sustainable forest management and increasing the resilience of Britain’s forests and woodlands. The genetic diversity within a tree species at any one time is the result of many dynamic processes, and it provides the source for future adapted trees and woodlands. Its importance is recognised in The UK Forestry Standard and forestry practitioners are encouraged to consider genetic diversity when managing forests and woodlands. One method of genetic conservation is to manage specific areas with the intention of allowing the full cycle of natural processes to occur. These areas are called gene conservation units. This Practice Note sets out what you need to do to establish a gene conservation unit and describes the recommended management approaches. Many woodlands may already be managed in a way that would make them suitable, but a more formal recognition of a network of gene conservation units allows for a more robust and quantifiable approach. The approach described here allows for a consistent method of selecting and describing units across the full range of a species and is compatible with the approach promoted across Europe.