It is of paramount importance to have an accurate and reliable fracture assessment code to reach an optimum design. Such an assessment enables design engineers to predict the exact conditions causing a cracked component to fracture. Therefore,lighter yet safer structures can be manufactured. The available fracture criteria implemented in the fracture assessment codes are not ideal by any means and need improvement. It has long been known that plastic constraint effects should be considered if a more accurate fracture criterion is to be proposed, yet there is still no widely accepted parameter to quantify it. This book is aimed to quantify the constraint level properly. After quantifying constraint for both in-plane and out-of-plane directions, a new fracture criterion is proposed which is capable of predicting fracture in any geometry, thickness and loading condition.