In this review the recent progress in the development of suitable precursors for ¹¹C-labelling is discussed. Especially the last few years' advancement of the use of [¹¹C] carbon monoxide as a versatile and useful precursor in labelling chemistry is presented. The development is set in perspective of its potential in applying molecular imaging tools in drug and tracer development.The possibility of exploring small tracer libraries utilizing the microdosing concept is explored.