Image fusion finds numerous applications in remote sensing, satellite imaging, medical imaging etc. In medical science, in order to diagnose a disease, it is required that the image obtained from a particular modality should be highly informative and should have high accuracy and also it should have high spatial as well as high spectral resolution. However, most of the available modalities alone are not capable of doing it convincingly. To solve this problem, a technique called image fusion has been evolved, in which two or more images are fused together to make a new image. In medical image fusion two or more images obtained from different modalities are fused together to give us a desired image. Selection of fusion rule should be such that it must provide us all the relevant information and at the same time does not introduce any undesired features to the resulting image. In this paper we have considered various image fusion techniques to be used as the performance metric evaluators for the research study. We trust that this review would turn into a reference in this consistently developing field of exploration.