Considering that the correct quantification of fatigue damage involves two load parameters, the available load ratio data on polymeric materials are analyzed. It is shown that crack growth can be characterized by two parameters, ΔK and K max, without the need for any crack closure concept. The crack growth rates, starting from the threshold can be represented by the L-shaped curves in the ΔK-K max plane defining two limiting variables, ΔK* and K max*. Crack growth trajectory maps for various materials are developed by plotting ΔK* versus K max*, as a function of crack growth rate. The trajectory defines the crack growth resistance curve providing a measure of material resistance to increasing crack tip driving forces.