This chapter departs from the classic definition of parties and applies a novel theory that casts doubt on the validity of the minimalist definition using two Costa Rican cases. Discussion of horizontal and vertical mechanisms as applied to the National Liberation Party (PLN) and Citizens Action Party (PAC) reveals strong similarities in terms of party organizations and challenges for policy consistency at the local level combined with marked contrasts in their capacities to process collective demands. The main finding of this chapter is that the PLN and PAC, two of the most prominent parties in the country, are quite different one from another. Regardless of their remarkable differences, both parties share the feature of having evolving constituencies, though their respective constituencies are evolving in opposite directions. The PLN fits the classic definition of a party with a well-organized and fully developed constituency suffering from decreasing membership over the last two decades. The PAC, on the other hand, does not have a sufficiently developed constituency to be considered a fully functioning party. The PAC does, however, have activists and is developing its membership.