This book provides some fundamental background of fuzzy set, intuitionistic fuzzy set, soft set and complex intuitionistic fuzzy set with their definitions and various properties. In addition to this, we have also presented applications of these theories in statistical regression analysis, decision making and reliability evaluation of a system along with a brief literature survey. Further, the study of new intuitionistic fuzzy information measures, similarity measures, fuzzy linear regression model, intuitionistic fuzzy reliability and complex intuitionistic fuzzy soft sets with their entropies have been presented. Computational applications of the given information measures in the field of pattern recognition and image thresholding has been well proposed with discussion.