This document reviews digital watermarking techniques for copyright protection of digital data using transform functions. It discusses how transform-based watermarking systems like discrete wavelet transform and discrete cosine transform are robust against many attacks but are vulnerable to geometric attacks like rotation and noise attacks. The document provides an overview of different watermarking types and reviews several related works on digital watermarking techniques. It also discusses various types of attacks on watermarks like subtractive attacks aimed at detecting and extracting the watermark and distortive attacks that apply transformations to degrade the watermark.