Authentication is process in which right user will be given access to resource. Authentication protects resource to access from unauthorised user. There various traditional techniques are available for authentication. But these techniques have disadvantages. To overcome this disadvantages multi-factor authentication is used for authentication. In multi-factor authentication more than one authentication method is combined to perform authentication. One form of authentication that is mostly used with other forms of authentication for multifactor authentication is one time password (OTP). One time password is valid for one login session. In this paper we conduct survey of existing one time password generation methods.