http://bitcoins.net/images/double-spend.jpg Through out the history money changed in shape and nature, and finally it becomes invisible, untouchable, weightless. Yes, I am talking about Electronic Money, Digital Money, Cryptocurrency.When money change it nature to be DIGITAL, what will happen? Everything which is DIGITAL can easily "COPY & PASTE" which means Duplicate, Triplicate, Multiply.... In the digital financial world we called this phenomena "**Double Spend Problem**" or "**Infinite Spending Problem**". https://moneyconnexion.com/wp-content/uploads/2014/07/copy_paste_job.jpg Alice could use 10$ to buy something from Bob at the same times when the confirmation haven't reached, she could use the same 10$ to pay for Charlie and make the confirmation first. By the time 10$ reaches Bob, it becomes an invalid transaction as it was already spent to Charlie. Within the distributed system, people try to create a Timestamp sever, where everyone in this planet agree the time of each transaction and has one global state view. https://marmelab.com/images/blog/double_spend.png Bitcoin is not the first that try to solve this problem, the concept of electronic cash is not new, since 1982 David Chaum developed untraceable digital currency using blind signature and later on developed E-cash. E-cash system could only detect the double spend problem but haven't solved it yet. In 1997, Adam Back developed another system called hashcash which was actually used to prevent spam in the email. The idea was to solve a computational mathematics puzzle which was hard to compute but easy to verify and later on became the Proof of Work (PoW) Concept. In 1998, B-Money was proposed by Wei Dai using proof of work mechanism to create digital currency, but the system didn't taking into account the consensus mechanism and the malicious action such as Sybil Attack. Nick Szabo also developed BitGold but also has the same problem as B-Money. https://steemitimages.com/DQmTUWMtYg43RHW9kS4iRsDS1Q78ZTSWF69SqyiTmxwR3X8/image.png In 2008, the foundation in the past has lead to a ground breaking discovery, the smart combination of the previous works has laid the strong foundation for the new disruptive technology to be born The Blockchain- Distributed Ledger Technology and the new type of digital currency bitcoin - cryptocurrency. By. @Cryptopreneur