Biological systems are invariably dynamic, with both stochastic interactions and deterministic processes across multiple timescales ensuring the maintenance of homeostatic regulation and allowing us to adapt to changes in both internal and external environments. It is no surprise therefore that the stress responsive hypothalamic-pituitary-adrenal (HPA) axis shows multiple levels of regulation which come together to regulate oscillating levels of glucocorticoid secretion with both diurnal and ultradian rhythmicity. I shall describe the mechanisms underlying the HPA pulsatility and how these interact with higher level circadian control by the hypothalamic suprachiasmatic nucleus. I will show how the adrenal adapts to pulsatile ACTH and how tissues respond to pulsatile changes in cortisol/corticosterone. The importance of this for optimal emotional and cognitive function in man will be described. Finally, I shall show how novel technology measuring dynamic changes in hormone levels in ambulatory subjects in their own homes/at work can be used to diagnose endocrine disease.