The measurement of the acoustic modes of the cosmic microwave background have been perhaps the most exciting new astrophysical territory explored in the 21st century. This cosmological area includes the study of some of the earliest moments of the Universe, such as the significant change in state of going from a gas of ionized particles to one of atoms (called recombination), greatly reducing the opacity for photons, leading to the observable phenomenon of the microwave background today. This chapter builds up to a calculation of the sound horizon and hence the location of the first observed peak in the fluctuation spectrum.