We assume that the reader is acquainted with the mathematical – statistical instrumentation and terminology referring to Portfolio Theory. We tried, without graphic illustration, to explain theory basis of portfolio using original papers of authors in this area. If investments within economy bring the most messages for the future, it comes out that “failed investments” in the form of wrong decisions contributed in the way of making in coming future to be worthless and to be late. The individual investor is motivated by expecting the return of investments and profit from investments, but there is the restraint in the risk. Portfolio theory helps in controlling the risk and return and reaching appropriate decision.