The problems that have to be addressed in designing suitable spin probes are manifold. In this article it will be shown that some the properties the probes need to possess are (i) a simple EMR spectrum (in some types of imaging or oximetry a single line is desirable), (ii) sharp lines with no unresolved splittings, to provide maximum sensitivity to detection, (iii) chemical, metabolic and thermal stability, (iv) easy possible synthesis to permit substitution with isotopes, prosthetic groups or groups to control the partition coefficients in lipid/water systems, and (v) electronic anisotropy, if rotational correlation times are to be determined, for example for the measurement of local viscosity.