In any consideration of long term (one or two years) human activities on Mars, it is imperative to evaluate the effects of the environment. There are two aspects of such environmental assessments: (1) hazardous parameters and (2) resource availability. This paper looks at both from the point of view of future human expeditions which include the requirement to explore Mars at some distance (e.g., several kilometers) from the landing site, and the ability to perform in situ measurements, physical and chemical analyses of surface and subsurface materials. Human activity on Mars clearly requires more support (food, water, air) than can be transported by a spacecraft. At the same time, resources for human protection from environmental hazards must be provided. This paper offers a discussion and analysis of these problems, based primarily on Mars fly-by, orbiter and lander data obtained in the 60's and 70's. The Viking orbiters and landers have been the most useful in that context.