Blockchain is the new “buzz” word that has attracted the attention of industries and businesses. It is an innovative technology that provides information exchange in an efficient and transparent manner. It has a wide range of application varying from cryptocurrency, healthcare, risk management, education, financial services, internet of things (IoT), border security to public services. However, security issues and threats of this novel technology is also an important topic. In this chapter, the authors provide a comprehensive study of applications, challenges, and issues and how to combat them in the blockchain. Major areas of concern are security, scalability, cryptocurrency's malicious attacks, etc.