This paper proposes a blind watermarking scheme using wavelet tree quantization. The wavelet coefficients of the host image are grouped into wavelet trees and each watermark bit is embedded using two trees. The trees are so quantized that they exhibit a large enough statistical difference, which can later be used for watermark extraction. Each watermark bit is embedded in all frequency bands, which renders the mark more resistant to attacks that remove certain frequency components. Also, the embedding is spread throughout a relatively large spatial region, which yields more robustness against spatial domain geometric attacks. Examples of various attacks are given to demonstrate the robustness of the proposed technique.