Ornamental trees refer to woody plants with high ornamental value in terms of leaves, stems, flowers, fruits, and tree shapes. In densely populated Hong Kong, ornamental trees can provide a variety of ecosystem services and greatly enrich the city's biodiversity. It brings valuable benefits in many aspects such as economy, society and public health. In addition to illustrating 100 species of ornamental trees, the book also explains the history and transformation of tree planting in urban areas of Hong Kong, the 2D Barcode Tree Labelling Scheme, and the Tree Viewing and Tree Photography Guidelines, so that readers can appreciate the beauty of trees and understand the ornamental trees that have contributed to the sustainable development of the city from multiple perspectives.