Intellectual property (IP) block reuse is essential for facilitating the design process of System-on-a-Chip. Sharing IP blocks in such a competitive market poses significant high security risks. IPs can be read, copied or even partitioned to cover the authorship proof. Creators and owners of IP designs want assurance that their content will not be illegally redistributed by consumers. Consumers, on the other hand, want assurance that the content they buy is legitimate. Digital watermarking, used with most of the shared digital media, has emerged as a candidate solution for helping copyright protection of IP blocks. In this paper, we outline IP watermarking and survey the current state of-the-art of different schemes and algorithms. We also highlight the main technical problems that should be solved in order to let IP watermarking be used widely in industry.