The Horn conjecture and the Littlewood–Richardson rule

In 1912 Hermann Weyl asked the following question: what can be said about the spectrum of the sum of two Hermitian matrices $A$ and $B$ with given eigenvalues? In 1962 Alfred Horn produced a list of inequalities on the spectra of $A$, $B$ and $A+B$, which he conjectured to be necessary and sufficient. This conjecture was proven by A. Klyachko, A. Knutson and T. Tao in 1999. Knutson and Tao proposed a nice combinatorial description of Horn's inequalities by certain combinatorial diagrams, known as honeycombs.
These diagrams are directly related to the Littlewood–Richardson problem of decomposing the tensor product of two irreducible representations of $GL(n)$ and to Schubert calculus on Grassmannians. They provide a new symmetric formulation of the Littlewood–Richardson rule by means of the so-called Knutson-Tao puzzles: tilings of a equilateral triangle by certain tiles. I will explain the relations among these problems and, time permitting, discuss some other problems featuring honeycombs and puzzles.