Planning started in the U.S. as an attempt to solve public health problems such as the cholera epidemics that engulfed many American cities in the late 1800s. Planners worked on providing city infrastructure (providing water, sewers and roads, and attempting to ease the overcrowding that bedeviled many American cities during this period). Over time, this evolved into a broader effort to regulate how, where and when building could occur, to make cities more pleasant and efficient places to live and work. Managing development is a major function of planning to this day (and is referred to as "land-use planning").
In the last half of the twentieth century, suburbs grew ever faster while most central cities shrank. Planners became concerned with the environmental and economic consequences of development. Reducing urban sprawl through growth controls, changing transportation and development patterns to reduce air and water pollution, and revitalizing distressed regions and inner city neighborhoods, became priorities. Planners couldn't do this alone: ensuring democratic citizen participation was essential if major policy changes were to succeed.
Despite many successes, it was soon clear that a more comprehensive approach was needed. Planners (and others) have now begun to think about the long-term environmental and economic sustainability of our development patterns. Designing "green" infrastructure, finding alternatives to new highway construction, ensuring everyone has access to affordable housing in livable neighborhoods, and stimulating the redevelopment of abandoned, contaminated "brownfields," are just a few examples of the sustainable alternatives planners work for.
To get a better sense of what planners do, go to the official American Planning Association (APA) web site. For more information on what planners in your state are doing, go to the state chapter links. APA also runs divisions devoted to specific sets of issues. For instance, the New Urbanism Division provides a home and resources for those interested in creating New Urbanist communities (new communities that have the look and feel of traditional American neighborhoods).
The Association of Collegiate Schools of Planning also provides information on planning schools, careers in planning and jobs.