Glass and graphite fiber reinforced injection molded thermoplastics contain very short fibers, resulting in matrix/interface dominated properties. Molded parts typically fail by propagation of a single dominant crack. A fiber avoidance crack growth model is presented in which the crack extends by the coalescence of isolated failure zones; the crack tip process zone dimension is fixed by the microstructure (fiber) size and so does not change with test conditions or Kmax in fatigue crack growth. Fracture toughness data for several materials correlate with this model. Fatigue crack growth follows a power law relationship with an exponent close to 4 for the pure matrices and 7 to 8 for the composites. The threshold KI values where cracks will not grow are a much higher fraction of the fracture toughness for the composites than for most unreinforced polymers or metals. The trends of S-N curves for unnotched molded bars follow the same exponent as observed in fatigue crack growth. Shifts in the pure matrix failure mode in fatigue are reflected in the local failure mode of the composites.