The purpose in writing this expository monograph has been three-fold. First, the author set out to present the solution of a problem posed by Wolfgang Krull in 1932. He asked whether what is now called the "Krull-Schmidt Theorem" holds for artinian modules. A negative answer was published only in 1995 by Facchini, Herbera, Levy and Vamos. Second, the author presents the answer to a question posed by Warfield in 1975, namely, whether the Krull-Schmidt-Theorem holds for serial modules. Facchini published a negative answer in 1996. The solution to the Warfield problem shows an interesting...