  For a graph $G$, the $k$-colour Ramsey number $R_k(G)$ is the least integer
$N$ such that every $k$-colouring of the edges of the complete graph $K_N$
contains a monochromatic copy of $G$. Let $C_n$ denote the cycle on $n$
vertices. We show that for fixed $k\geq2$ and $n$ odd and sufficiently large,
\[ R_k(C_n)=2^{k-1}(n-1)+1. \] This resolves a conjecture of Bondy and
Erd\H{o}s [J. Combin. Th. Ser. B \textbf{14} (1973), 46--54] for large $n$. The
proof is analytic in nature, the first step of which is to use the regularity
method to relate this problem in Ramsey theory to one in nonlinear
optimisation. This allows us to prove a stability-type generalisation of the
above and establish a surprising correspondence between extremal $k$-colourings
for this problem and perfect matchings in the $k$-dimensional hypercube $Q_k$.
