Progress in the study of deformation and strength in plastics has been strongly bound to a scientific framework, although much technological advance has succeeded by regular excursions from this base. Deformation theories have passed through linear elasticity to linear viscoelasticity and at present dwell in nonlinear viscoelasticity, but many practical problems remain unresolved by formal methods. The prediction of stiffness in practical mouldings through the adoption of traditional science is often too cumbersome. However ad hoc methods can be very successful, for example in the case of sandwich foam mouldings. Also fatigue strength in plastics relies heavily on empiricism. The authors believe that ad hoc methods will further the exploitation and perhaps understanding of deformation and strength properties in plastics