Although Bitcoin payments are often described as anonymous, they are really pseudonymous: the identity of the owner of a given address may not be known, but there is a permanent record of every transaction into and out of every account.
Every bitcoin transaction is traceable back to the block from which it was mined, as well as to every other address with which transactions have occurred. Every transaction is time-stamped, and the entire blockchain can be viewed by anyone.
While no technical solution can solve all aspects of the problem, especially against a determined attacker with almost unlimited resources, there are methods than can be used to protect the privacy of cryptocurrency users.
Centralized mixing services
Mixing services combine coins from many different sources before sending them back to their original owners (usually for a fee), with the intention of obscuring where the funds originated. In their simplest form, mixers are operated by individuals and users are simply required to trust them to return their funds. This is particularly unsatisfactory since they are often operated anonymously. More sophisticated forms of mixing have been automated and incorporated into the protocols of different cryptocurrencies. 
Peer-to-peer tumblers appeared in attempt to fix the disadvantages of centralized model of tumbling. These services act as a place of meeting for bitcoin users, instead of taking bitcoins for mixing. Users arrange mixing by themselves. This model solves the problem of stealing, as there is no middleman. Such protocols as Coin Join, SharedCoin and CoinSwap allow few bitcoin-users to gather in order to form one bitcoin exchange transaction in several steps. When it is completely formed, the exchange of bitcoins between the participants begins. Apart from mixing server, none of the participants can know the connection between the incoming and outgoing addresses of coins. This operation can be carried out several times with different recipients to complicate transaction analysis. Mixing using protocols such as CoinJoin ‘obfuscates money flows, and not account balances’, meaning that information on and off the blockchain can still be analysed to build up a picture of accounts usage and owner identity.
Famous mixing services
Most of bitcoin mixers provide service only in TOR network and require TOR Browser to reach them.
- Pete Rizzo (Sep 18, 2015). https://bravenewcoin.com/assets/Whitepapers/Bitcoin-DarkPaper.pdf. Retrieved 22 Apr 2017.
- Jon Matonis (Mar 17, 2014). "A Taxonomy of Bitcoin mixers". http://www.coindesk.com/taxonomy-bitcoin-mixing-services-policymakers/. Retrieved 23 March 2017.
- Dark Paper BTCD 1 (Aug 23, 2016). "Bitfury Research Shines Light on Bitcoin Privacy Weaknesses". http://www.coindesk.com/bitfury-research-seeks-shine-light-bitcoin-mixing-methods/. Retrieved 26 March 2017.