Blockchain technology ensures transparency and decentralization, but these two properties may also create serious privacy issues. This is mainly due to the fact that anyone can analyze transactions, follow users, and sometimes try to link blockchain addresses to real identities. Additionally, data stored on-chain cannot be deleted and smart contracts may reveal sensitive information if they are not carefully checked before deployment. In this paper, we present the main privacy issues identified in blockchain systems and the techniques commonly used to mitigate them. In addition, we briefly discuss the limitations of these techniques and the challenges that must be addressed to improve privacy in blockchain.