Millimeter-VLBI observations of blazars

High angular resolution observations of blazars are a key step to understand the properties of relativistic jets emerging from Active Galactic Nuclei (AGN). Space-VLBI and mm-VLBI are complementary approaches to obtain an extraordinary resolution, with the latter being more suitable in the case of the most compact self-absorbed regions at the base of the jet. In this contribution, I present the results of mm-VLBI observations of a TeV active blazar (Mrk501), successfully detected with a flux density of ∼ 110 mJy and a resolution of 110 μas x40 μas. A limb-brightened structure seems to be present even on the smallest scales. The need for new observations of Mrk 501 and of four more EGRET blazars is discussed, with an outlook to the main questions in the era of gamma-ray satellites.