I will explain the problem behind codes. When sending and receiving information, errors can happen. One wants to reconstruct the original information from the received. This leads to the concept of error correcting codes. This is so simple that one wonders why not everything is known. To indicate that this is completely wrong I will explain a relation between so called self dual codes and unimodular lattices, which lead to very difficult problems in analysis. On the way very important lattices like E8 or the Leech lattice will occur. Thus codes are very complicated. In such a situation it might be of interest if codes occur in a completely different mathematical context. I will describe how codes occur in topology and present results about the question, which codes come from topology.