Consider a pay TV provider serving countless users, who concurrently want to watch a live transmission of a soccer world championship. How can the provider restrict access to authorized users? Conventional access control systems, where users are supposed to log in and authenticate themselves, will likely be overloaded. This book introduces Electronic Subscriptions as a practicable solution for addressing the mentioned problem by using cryptographic primitives. It provides a solid theoretical background and gives practical solutions for such scenarios. Several questions are investigated, among others: How to ensure that only legitimate users are addressed? How to limit the validity of authorizations (e.g. year, month etc.)? How to revoke authorized users?