In the previous chapters, we showed how to compute every two-party (or multi-party functionality, but only for the coin-tossing case) while minimizing the interactions between the parties. Unfortunately, minimizing the round complexity does not necessarily lead to a more efficient protocol. Indeed, it is easy to see that the protocols shown so far are far from being practical. In this chapter, we aim to design a practical protocol, for a functionality that has a lot of relevance in practical applications: Private Set Intersection (PSI). In PSI two parties hold two sets of strings X and Y, respectively. At the end of the protocol one (or both) party should learn the intersection of the two sets \(Z= X\cap Y\) and nothing else about the input of the other party.