The escalation of ongoing research on the topic of conversion coatings for magnesium alloys confirms that the search continues for appropriate replacements to toxic chromate-based coatings. This chapter reviews the performance of the range of typical conversion coating technologies presently available. As a means to rationalise coating performance and characteristics more generally, thermodynamic analysis has been used to compare coating processes and coating pretreatment, the latter of which appears to be at least as significant as the choice of coating technology itself.