The study of the extragalactic background radiation has a long and honorable history. Landmarks include the discussion by de Vaucouleurs (1949) of the extragalactic contribution to the mean optical surface brightness of the sky (de Vaucouleurs' number, S10 ~ 0.5 tenth magnitude stars per square degree, is quite close to modern estimates); the discovery by Giacconi et al. (1962) of the X-ray background radiation; and the discovery by Penzias and Wilson (1965) of the microwave background radiation. It is only in the last decade that the subject has reached the very rich level you will find in the contributions in this volume, however. My comments on the state of the subject are organized by physical issues, and I use order-of-magnitude estimates in an attempt to illustrate the significance of the observations of the background fields for these issues. In some cases we do not know how to go much beyond the orders of magnitude, while in others detailed analyses are feasible and significant, as you will see in the other papers in this volume.