The managers of private forests have critical roles in responding to forest health challenges. Basic knowledge of how they interpret and understand the health of their trees is, however, very sparse. Via an online survey of private forest managers in Britain, we reveal the basic characteristics of this group's understandings of tree and forest health both in general and, more specifically, in relation to the health (and ill health) of oak species. The survey generated 441 valid responses – from forest owners, forestry agents and other professionals. The majority of our respondents believe the forest(s) they manage to be healthy or very healthy, although vulnerable to future threats. We identify key themes within managers' understandings of the factors that promote tree health, and describe the visual signs of good- and ill-health used by them as indicators. Oak trees and oak woodlands were similarly judged to be healthy currently, but again this positivity was set against a sense of vulnerability and potential future decline. The biggest threat to oaks was considered to be browsing and bark stripping by mammals – a very long standing concern – rather than more recently emergent threats. Through this research we provide a clear sense of how forest managers think about, and make judgements on, the health of their trees and forests. This understanding needs to inform the increasing efforts by the forestry sector in engaging private forest managers in forest protection.