It is believed that Mars underwent drastic climate change, changing its environment from warm and wet to cold and dry. This gives rise to the idea that Mars may have hosted life in the past and, indeed, may do so even today. Atmospheric evolution is thus an important key to understanding the history of Martian habitability. However, precise estimates of past atmospheric inventories including water, and their loss mechanisms, are difficult to be obtained. Recent studies have highlighted various interesting facts related to (i) the efficiency of mass transport from the lower to upper atmospheric reservoir and (ii) the deep energetic particle precipitation into the atmosphere from space. These new insights tell us that Mars is a mutually coupled system comprising the planet’s surface, lower and upper atmospheres, and the surrounding space environment. These relationships potentially imply an upward revision of the estimate of total atmospheric loss to space. Another relevant issue relates to the indirect signs of life in the Martian atmosphere. Scientists are particularly intrigued by clear evidence of a biological/geological signature, such as methane (CH4) in the Martian atmosphere. Although the presence of CH4 is still under debate because of large measurement uncertainties, the forthcoming ESA-Roscosmos mission, which employs the Trace Gas Orbiter (TGO), will settle questions on the existence of this gas and its origin.