The breadth of expression in singing depends on fine control of physiology and acoustics. In this review, the basic concepts from speech acoustics, including the source-filter model, models of the glottal source and source-filter interactions, are described. The precise control, the extended pitch range, the timbre control and, in some cases, the uses of alternate phonation modes all merit further attention and explanation. Here we review features of the singing voice and the understanding that has been delivered by new measurement techniques. We describe the glottal mechanisms and the control of vocal tract resonances used in singing. We review linear and nonlinear components of the voice and the way in which they are measured and modelled and discuss the aero-acoustic models. We conclude with a list of open questions and active fields of research.