The book explains concept and principle of heat sealing and the correlation between molecular properties and seal performance. First, theoretical aspects of heat sealing and the mechanism involved in seal formation are discussed. Then different test methods for evaluating seal and hot tack performance and quality are reviewed. Different common sealant materials in fl exible packaging are discussed including multiphase and bioplastic sealants. Effect of processing condition and material characteristics on seal and hot tack performance of sealant materials are reviewed. Mathematical models are presented as powerful tools to optimize sealing condition in order to balance production cost and seal properties. Finally, practical examples of sealant design are introduced in food and industrial packaging.