Some aspects of large-$N$ theories

This is a pedagogical review of some aspects of quantum field theories in the limit in which the number of internal degrees of freedom is large. The focus is on large-$N$ QCD. The authors briefly discuss several well-known approaches to a solution of the $N=\infty$ limit: loop equations, classical actions, and master fields. Eguchi-Kawai models are discussed in detail, and some recently obtained numerical results are reviewed.