Daily mean atmospheric pressure at the two Viking landers are presented for slightly more than a martian year. The seasonal variation of pressure owing to exchange of CO2 with the polar caps is quite evident and contradicts, in part, earlier theoretical results. Day-to-day variations are the result of passage of synoptic-scale high and low pressure systems and are an important clue to the general circulation of the atmosphere. The effects of global dust storms on the general circulation and on the diurnal variation of pressure are detected and interpreted.