The principles underlying the detection of methaemoglobin are discussed elsewhere, as are the properties of methaemoglobin. This chapter focuses on the reasons as to why one might have ended up with oxidized haem iron, and how to get out of such a situation. As with all such "why is there too much of substance x" questions, the answer is usually either excessive synthesis, or ineffective clearance. Both mechanisms need to be explored. In the interest of rapidly offering a brief summary to those intolerant of tangential gibberish, a table is available as a list of the major culprits.