This chapter deals with the problem of reliable transmission of digitally encoded information through an unreliable channelChannelunreliable. When we transmit information from a satellite, or an automatic station orbiting the moon, or from a probe on Mars, then for many reasons (e.g., sun-bursts) our message can be distorted. Even the best telecommunication systems connecting numerous information centres in various countries have some nonzero error rate. These are examples of transmission in space. When we save a file on a hard disc and then try to read it one month later, we may find that this file have been distorted (due to, for example, microscopic defects of the disc’s surface). This is an example of transmission in time. The channels of transmission in both cases are different but they have one important feature in common: they are not 100% reliable. In some cases even a single mistake in the transmission of a message can have serious consequences. We will show how algebra can help to address this important problem.