This text contains the proceedings of an IUTAM Symposium on turbulent flows at low Mach number, in which fluctuations in density occur as a result of heat transfer or of mixing of different fluids. Such flows occur in the atmosphere, the ocean, and in chemical engineering. In addition during combustion large fluctuations in density occur as a result of heat release; the flows that are considered here do not involve heat release, for the most part, but the variable density flows that are considered can serve as a paradigm for many of the problems that arise in combustion as a result of the density fluctuations. Compressible flows - such as supersonic ones - are also considered in order to underline their similarities to and their differences from low-speed variable density flows. The book should be of interest to mechanical, aerospace, civil, and chemical engineers, meteorologists and oceanographers and those who deal with fluids in which fluctuations of density occur.