The significance and even the existence of crack closure is being questioned by several researchers. The objective of this study was to determine if crack closure occurs and to quantify its significance. An approach combining experimental measurement techniques with finite element analysis techniques was utilized. For two values of compact tension specimen thickness, a series of tests were conducted to determine the effect of maximum stress intensity, load ratio, constraint, and single tensile overload on the crack closure and fatigue crack growth behavior of a modified 1070 steel. Test results indicated that constraint has a significant influence on crack closure and crack growth rate behavior. Thin specimens exhibited consistently lower crack growth rates and higher crack closure levels than the thick specimens, except for tests conducted at a high load ratio, where crack closure did not occur. The thin specimens also exhibited a more significant overload effect. A new finite element modeling technique, which uses substructuring techniques to model the load cycling and crack propagation of an entire compact tension specimen, was developed. Comparison of stationary crack and propagating crack finite element models revealed that plasticity-induced crack closure produces a significant amount of crack tip shielding, which effectively reduces the strain range and mean strain experienced at the crack tip.