The chapter devotes to defining the digital signature (DS), its trends, relevance, the security services provided by it, and the various schemes associated with DSs as well as some of the relevant applications of DS. In 1976, Whitfield Diffie, an American cryptographer, and Martin Hellman, an American cryptologist, first described the concept of DS strategy. A DS represents an electronic correspondent of "written signature." Undeniable DS scheme is another approach that comprises three major parts: a signing algorithm, a verification protocol, and a disavowal protocol. The major application of DS is in signing the certificates issued by the certification authority. DS algorithm (DSA) represents a federal information processing standard for the DSs. The advantage of DS is what makes DS as a viable replacement to wet ink signatures. Such technologies are quite helpful and popular in the global DS software market and quite beneficial while maintaining message non-repudiation, message authentication as well as message integrity.