A Reeb graph (named after Georges Reeb by René Thom) is a mathematical object reflecting the evolution of the level sets of a real-valued function on a manifold. A similar concept was introduced by G.M. Adelson-Velskii and A.S. Kronrod and applied to analysis of Hilbert's thirteenth problem. Proposed by G. Reeb as a tool in Morse theory, Reeb graphs are the natural tool to study multivalued functional relationships between 2D scalar fields ${\displaystyle \psi }$, ${\displaystyle \lambda }$, and ${\displaystyle \phi }$ arising from the conditions ${\displaystyle \nabla \psi =\lambda \nabla \phi }$ and ${\displaystyle \lambda \neq 0}$, because these relationships are single-valued when restricted to a region associated with an individual edge of the Reeb graph. This general principle was first used to study neutral surfaces in oceanography.

Reeb graphs have also found a wide variety of applications in computational geometry and computer graphics, including computer aided geometric design, topology-based shape matching, topological data analysis, topological simplification and cleaning, surface segmentation and parametrization, efficient computation of level sets, neuroscience, and geometrical thermodynamics. In a special case of a function on a flat space (technically a simply connected domain), the Reeb graph forms a polytree and is also called a contour tree.

Level set graphs help statistical inference related to estimating probability density functions and regression functions, and they can be used in cluster analysis and function optimization, among other things.

Given a topological space X and a continuous function f: X → R, define an equivalence relation ~ on X where p~q whenever p and q belong to the same connected component of a single level set f⁻¹(c) for some real c. The Reeb graph is the quotient space X /~ endowed with the quotient topology.

Generally, this quotient space does not have the structure of a finite graph. Even for a smooth function on a smooth manifold, the Reeb graph can be not one-dimensional and even non-Hausdorff space.

In fact, the compactness of the manifold is crucial: The Reeb graph of a smooth function on a closed manifold is a one-dimensional Peano continuum that is homotopy equivalent to a finite graph. In particular, the Reeb graph of a smooth function on a closed manifold with a finite number of critical values –which is the case of Morse functions, Morse–Bott functions or functions with isolated critical points – has the structure of a finite graph.

Let ${\displaystyle f:M\to {\mathbb {R} }}$ be a smooth function on a closed manifold ${\displaystyle M}$. The structure of the Reeb graph ${\displaystyle R_{f}}$ depends both on the manifold ${\displaystyle M}$ and on the class of the function ${\displaystyle f}$.

Since for a smooth function on a closed manifold, the Reeb graph ${\displaystyle R_{f}}$ is one-dimensional, we consider only its first Betti number ${\displaystyle b_{1}(R_{f})}$; if ${\displaystyle R_{f}}$ has the structure of a finite graph, then ${\displaystyle b_{1}(R_{f})}$ is the cycle rank of this graph. An upper bound holds