||In this thesis, we are concerned about data hiding/watermarking for halftone images. Halftone images contain only 2 tones and they are generated from multi-tone images by halftoning techniques such as ordered dithering and error diffusion. Image data hiding means hiding or embedding of invisible data in an image without affecting its perceptual quality, such that the hidden data can be extracted with certain procedure. The study of data hiding techniques is commonly called watermarking. The core idea of this thesis is to combine watermarking and halftoning to generate watermarked halftone images from multi-tone image directly. In this thesis, four algorithms for different applications are developed, which are Data Hiding by Stochastic Error Diffusion (DHSED), Data Hiding by Conjugate Error Diffusion (DHCED), Correlation-based Watermarking Error Diffusion (CWED) and Data Hiding Ordered Dithering (DHOD). DHSED and DHCED can embed binary image in halftones. The binary image can be viewed or detected through overlaying or Boolean operation. CWED is capable to embed robust binary data in halftones. DHOD is a high capacity data hiding method developed particularly for ordered dithering.