Over a third of patients with angina have non-obstructive coronary arteries (ANOCA) on diagnostic coronary angiogram. There is a group of these individuals, comprising predominantly of women with angina, where the defining features include the phenomenon of lingering symptoms after a trigger has long passed, and variable responsiveness to glyceryl trinitrate spray. Many but not all have objective evidence of cardiac ischaemia with non-invasive tests, and in some, pacing stress can cause lactate spillover in the coronary sinus. Excluding obvious vasoreactivity with spasm testing, they were previously labelled as cardiac syndrome X, but are now reclassified as coronary microvascular dysfunction (CMD). Diagnosed as reduced coronary flow reserve (CFR), or the inability of the heart to double the hyperaemic coronary flow, that is, a CFR < 2.0. This chapter describes 3 selective cases of CMD diagnosed with coronary bolus and continuous thermodilution and postulates on their varied mechanisms which include both extra- and intra-cardiac constraints on the myocardial microvasculature.