In this section, just as in the rest of Chapter 8, all point sets are assumed to be in general position, that is, in the plane, they do not contain three points on a line, or, in d-dimensional space, d + 1 points in a hyperplane. These point sets can be described by their order types, as defined in Section 8.1.Perhaps the most famous problem concerning point sets in general position is the "Erdős-Szekeres convex polygon problem," also known as Esther Klein's problem or the Happy End problem* (see [MoS00], [BáK01], for recent surveys). What is the smallest number f ES (r) such that any set of f ES (r) points in general position in the plane contains r points that form the vertex set of a convex r-gon? Instead of saying that a point set contains the "vertex set of a convex r-gon," in the sequel we simply say that it contains a "convex r-gon."In their first joint paper, Erdős and Szekeres [ErS35] established the existence of this number by reducing the result to a Ramsey-type statement and rediscovering Ramsey's theorem for its proof. In a much later paper [ErS60], the same authors obtained the lower bound f ES (r) ≥ 2 r−2 + 1. The only known exact values of this function are f ES (3) = 3, f ES (4) = 5, and f ES (5) = 9 [Bo74], [KaKS70]. It is conjectured that the lower bound is sharp for all r. Conjecture 1 (Erdős-Szekeres [ErS35]) Any set of 2 r−2 + 1 points in general position in the plane contains a convex r-gon.