Carbon Dioxide: Friend or Foe is a short rnonograph on the so-called carbon dioxide greenhouse effect. The author challenges the established view that the present CO2 increase would, in the long term, lead to a global ground temperature increase. S. B. Idso, from four sets of observations, has deduced that the temperature response to an increased received energy at the ground should be less than or equal to 0.113 K (W/m2). If this result is combined with the 2.28 W/m2 of increased radiation expected from CO2 doubling, he finds a temperature increase of 0.26 K, which cannot be distinguished form the natural temperature fluctuation. This conclusion is in disagreement with virtually all the current mathematical models that predict a ground temperature response of an order of magnitude or more higher.