In this article, we prove the mean convex neighborhood conjecture for the mean curvature flow of surfaces in R 3 . Namely, if the flow has a spherical or cylindrical singularity at a space-time point X = (x, t), then there exists a positive ε = ε(X) > 0 such that the flow is mean convex in a space-time neighborhood of size ε around X. The major difficulty is to promote the infinitesimal information about the singularity to a conclusion of macroscopic size. In fact, we prove a more general classification result for all ancient low entropy flows that arise as potential limit flows near X. Namely, we prove that any ancient, unit-regular, cyclic, integral Brakke flow in R 3 with entropy at most 2π/e + δ is either a flat plane, a round shrinking sphere, a round shrinking cylinder, a translating bowl soliton, or an ancient oval. As an application, we prove the uniqueness conjecture for mean curvature flow through spherical or cylindrical singularities. In particular, assuming Ilmanen's multiplicity one conjecture, we conclude that for embedded two-spheres the mean curvature flow through singularities is well-posed. 1 2 KYEONGSU CHOI, ROBERT HASLHOFER, OR HERSHKOVITS 5.2. Cap size control and asymptotics 51 6. Rotational symmetry 53 6.1. Fine expansion away from the cap 53 6.2. Moving plane method 57 7. Classification of ancient low entropy flows 61 7.1. The noncompact case 61 7.2. The compact case 64 8. Applications 65 8.1. Proof of the mean convex neighborhood conjecture 65 8.2. Proof of the uniqueness conjectures for weak flows 70 References 71