Forest is in trouble. The most recent (2015) FAO Forest Resources Assessment shows an encouraging trend towards a decrease in deforestation rates, but it also points out that since 1990 total forest loss corresponds to an area the size of South Africa. Efforts to curtail deforestation require reliable assessments, yet current definitions for what a forest exactly is differ significantly across countries, institutions and epistemic communities. Those differences have implications for forest management efforts: they entail different understandings about where exactly a forest starts and ends, and therefore also engender misunderstandings about where a forest should start and end, and about how forests should be managed. This special issue brings together different perspectives from practitioners and academic disciplines - including linguistics, geographic information science and human geography - around the problem of understanding and characterizing forest. By bringing together different disciplinary viewpoints, we hope to contribute to ongoing interdisciplinary efforts to analyse forest change. In this introduction, we propose that interrogating the relationship between forest definitions, boundaries and ways of valuing forests constitutes a productive way to critically conceptualize the trouble that forest is in.