Distributed shared memory (DSM) systems have attracted considerable research efforts recently, since they combine the advantages of two different computer classes: shared memory multiprocessors and distributed systems. The most important one is the use of shared memory programming paradigm on physically distributed memories. One possible classification taxonomy, which includes two basic criteria and a number of related characteristic, is proposed and described. According to the basic classification criteria-implementation level of DSM mechanism-systems are organized into three groups: hardware, software, and hybrid DSM implementations. The paper also presents an almost exhaustive survey of the existing solutions in an uniform manner, presenting their DSM mechanisms and issues of importance for various DSM systems and approaches.