Explicit construction of families of linear expanders and superconcentrators is relevant to theoretical computer science in several ways. There is essentially only one known explicit construction. Here, a correspondence is shown between the eigenvalues of the adjacency matrix of a graph and its expansion properties, and this result is combined with results on group representations to obtain many new examples of families of linear expanders. Better expanders than those previously known are obtained and used to construct explicitly n-superconcentrators with congruent 157. 4n edges, much less than the previous most economical construction.