Temperatures are rising and the thermal regimes of our ecosystems are changing rapidly, becoming warmer and more variable with more frequent extreme events. The impacts of these changes are the subject of much current research on a wide variety of organisms. Important questions revolve around what determines thermal tolerance or the thermal limits of a species distribution. Are they the same thing? And how best to measure or predict them? To assess the integrated response of organisms and communities to changing thermal regimes requires systems level perspectives that draw together impacts at different scales – e.g., cell vs organ vs organism within species; or plant vs pollinator vs pathogen at community scales. I will present work collaborators and I have been doing to better understand what thermal tolerance means, what drives it, and to explore how best to assess it.