Image registration has become essential in several domains where reliable acquisition of aligned images cannot be assured  or turns out to be complex. Needless to mention, the significance of this problem is made most apparent when alignment of a group of images must be guaranteed and the images are rather different in nature although they describe the exact same thing.
Misalignment can be due to movement of the object or objects of interest, change of view-point, change in general conditions at the scene and even morphic changes. Morphic changes can be observed over time in the scene or within its constituent parts, e.g. the inevitable change in the form of the lungs. In some circumstances, as later discussed, misalignment is due to profound changes in the form of objects (usually subjects) being scanned. Alignment is a key step that must be completed before analysis of the joining of images is safely embarked on. It allows better understanding of the contents of all images as a group - something which is hard to achieve in the lack of consistency.