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This chapter introduces the specific biological and ecological elements of trees that need to be considered in the risk assessment of GM (genetically modified) trees, and the basics of environmental risk assessment. Trees have perennial growth habits with long lifespan, and many pathways for dispersal of propagules. Moreover, trees are often key species in ecosystems, they are present from temperate to tropical climates and have complex and multi-level interactions with other organisms. The possible environmental impacts of GM trees are discussed, and examples of scenarios where the GM trees may disperse and have environmental impacts are given. The chapter describes different environmental risk assessment frameworks. The basics of any risk assessment are hazard identification, estimation of potential consequences, estimation of the likelihood of these consequences to occur, estimation of the overall risk and deciding about risk management measures. Carrying out risk assessment usually includes a problem formulation step, definition of assessment endpoints from the broad protection goals and identification of those attributes that will be measured (measurement endpoints). As examples the EFSA guidance on risk assessment of GM plants and the Cartagena Biosafety Protocol guidance of GM trees are described. Moreover, both the comparative and per se risk assessment approaches are discussed.