Network coding is an elegant and novel technique introduced at the turn of the millennium to improve network throughput and performance. It is expected to be a critical technology for networks of the future. This tutorial addresses the first most natural questions one would ask about this new technique: how network coding works and what are its bene- fits, how network codes are designed and how much it costs to deploy networks implementing such codes, and finally, whether there are meth- ods to deal with cycles and delay that ar e present in all real networks. A companion issue deals primarily with applications of network coding. e present in all real networks. A companion issue deals primarily with applications of network coding.