In this thesis we present several results in coding theory, concerning error-correcting codes and the Shannon capacity. 1. We give a general symmetry reduction of matrices occuring in semidefinite programs in coding theory. 2. We apply the symmetry reduction to efficiently compute semidefinite programming upper bounds for nonbinary error-correcting codes equipped with the Hamming distance (joint work with Bart Litjens and Lex Schrijver), with the Lee distance and for binary constant weight codes. 3. We explore other methods to find new upper bounds for nonbinary codes with the Hamming distance, based on combinatorial divisibility arguments. 4. We study uniqueness and classification of codes, using the output of the semidefinite programming solver. Most of our classification results are related to subcodes of the binary Golay code. This is joint work with Andries Brouwer. 5. We consider the Shannon capacity of circular graphs. The circular graph $C_{d,q}$ is the graph with vertex set $\mathbb{Z}_q$ (the cyclic group of order q) in which two distinct vertices are adjacent if and only if their distance (mod $q$) is strictly less than $d$. The Shannon capacity of $C_{d,q}$ can be seen to only depend on the fraction q/d. We prove that the function which assigns to each rational number $q/d$ the Shannon capacity of $C_{d,q}$ is continuous at integer points $q/d$. Moreover, we give a new lower bound on the Shannon capacity of the $7$-cycle. This is joint work with Lex Schrijver. This thesis was written at the University of Amsterdam under supervision of Lex Schrijver.