In this chapter we look at the issue of sending messages over very large distances using electromagnetic radiation. An interstellar spacecraft is likely to have a small, low-power transmitter. Diffraction will reduce the intensity received at the Earth to tiny amounts, making the signal hard to detect even with large receivers. This limits the achievable data rates significantly. We see how to compute the power of the received signal, how to modulate messages into electromagnetic radiation, and how to use signal redundancy – error-correction coding – to reduce information loss due to noise. The received power may be so low that reception occurs in the regime of individual photon counting; we examine how many bits of information photons can carry. We will also examine the choice of wavelength – in particular optical vs radio – and the relative merits of placing receivers on the Earth compared to in space.