Conventional level of confidence called level of probability, which is also called Confidence Range is described in this chapter. Major principles of Confidence Interval are enumerated after defining Null Hypothesis and True Mean and Confidence Interval. Confidence Interval on True Mean and True Proportion are illustrated with examples. Special note is emphasised on two independent means as well as paired means in case of Confidence Interval. Chapter is concluded with a special note on Confidence Interval estimation. With relevant questions and answers, author makes the reader to study and learn from selected problems, collected from medical college and hospital studies. Chapter is concluded with glossary of the chapter.