As protein databases continue to grow in size, exhaustive search methods that compare a query structure against every database structure can no longer provide satisfactory performance. Instead, the filter-and-refine paradigm offers an efficient alternative to database search without compromising the accuracy of the answers. In this paradigm, protein structures are represented in an abstract form. During querying, based on the abstract representations, the filtering phase prunes away dissimilar structures quickly so that only a small collection of promising structures are examined using a detailed structure alignment technique in the refinement phase. This article reviews mainly techniques developed for the filtering phase.