Until quite recently, the theory of continuous media dealt mainly with diamagnetic and paramagnetic fluids. The researchers did not consider strongly magnetizable fluids, probably because such media had not yet been discovered in natural conditions and did not exist until recently as manufactured products. However, in the early 1960 s, magnetic fluids (MF) could be synthetically prepared, and then, technological interest in MF grew rapidly. Besides, magnetic refrigeration has been used for over 60 years as a technology to achieve temperatures below 1 K, employing the magnetocaloric effect of some solid materials. However, in the last 20 years, the technology has been developing for refrigeration applications above 1 K up to and including heat pumps above room temperature. The work has been multinational in scope and has focussed on the analysis of magnetic thermodynamic systems, investigation of magnetic materials suitable for refrigerants and development of prototype refrigerators. After a synthetic review of the basic properties of MF, of magnetic refrigerators and heat pumps and of magnetic properties of living matter, the aim of this paper is a thermodynamic examination of these systems in comparison with the relations usually applied to fluids. Three independent variables are generally considered, whereas usually only two variables have been considered until now for what is known to the authors. Also, in the case of only two independent variables, some results are obtained which are original. Furthermore, some applications of MF are taken into account, in particular, possibilities of converting thermal into mechanical energy are examined.