An approach for predicting trends in the toughness of particulate filled polymers has been presented. The approach is based on independent knowledge of the constitutive law that describes the non-linear behaviour in the process zone. An idealized law is used to demonstrate expected trends with particulate volume fraction and size. The trends are correlated with experimental data. Some discussion of the non-linear process zone mechanisms, such as debonding and microcracking, is presented as a basis for developing more realistic constitutive laws and hence, providing superior predictions of toughness.