The growth in applications of natural fibre polymer composites (NFPCs) has increased the importance of understanding time-dependent viscoelastic properties such as creep resistance, stress relaxation, and fatigue, all of which are covered in this chapter. The fundamentals of measurement of these properties are discussed, and the use of short- and long-term prediction of creep resistance of the NFPCs to achieve adequate long-term performance is outlined. The effects of the interfacial interaction between natural fibres and polymers on fatigue and stress relaxation properties of the NFPCs are explored. © 2011 Woodhead Publishing Limited. All rights reserved.