The features of the ideal elastic-plastic stress-strain curve compared with practical curves are discussed with reference to conventional materials engineering and geosynthetic materials testing practices. Uniaxial and biaxial test curves are compared and referenced to the modes of failure of the two types of test specimens. A comparison of the well-defined yield points in HDPE geosynthetics with the smoother elastic-plastic transitions in LLDPE and PP is presented, together with methods that define artificial yield points. Considerations for the use of one, rather than two, gage length values for the determination of yield and break strain properties during uniaxial tension tests are presented. Plane stress and plain strain behavior of geosynthetics in laboratory uniaxial tests and in the field are compared together with their significance in the stress crack testing and elongation performance in HDPE geomembranes. Reference is also made to the measurement of strain and its significance during the shear testing of seams.