As main memory capacities grows larger and larger, the memory era has arrived and inmemory databases have taken the place of traditional diskbased databases to provide efficient data management. In this paper, we analyzed the fundamental elements in inmemory index designing: summarized and evaluated the existing index structures, pointing out the future opportunities and challenges based on the development trend of current applications. Finally, we introduced our ongoing distributed inmemory index studies on the Cluster Aware InMemory System (CLAIMS).