This chapter tackles the simple problem of intersecting two (sorted) lists of increasing integers, which constitutes the backbone of every query resolver in databases and (Web) search engines. In dealing with this problem, the chapter describes several approaches of increasing sophistication and elegance, which eventually turn out to be efficient/optimal in terms of time and I/O complexities. A final solution will deploy a proper compression of the input integers and a two-level scheme aimed at reducing the final space occupancy and working efficiently over hierarchical memories.