Problem Based Learning is a strategy that builds a curriculum around a central question.  The question may force the student to either solve a problem or make a decision.  Some refer to this focussing question as an "Essential Question" because it centers the inquiry on the essence of an issue or topic.  Attributes of a good essential questions are as follows:

  it puts the student in the role of an investigator,
  requires the generation (by the student) of subsidiary questions,
  builds on, or drives the student to a solid knowledge base,
  has the potential to generate a variety of reasonable answers rather than closing
      in on a single "right answer," or any indisputable answer,
  is intriguing and socially relevant to the student,
  designed around solving a problem or making a decision,
  tends to become interdisciplinary when thoroughly pursued,
  drives students to the higher levels of Bloom's Taxonomy.

1. Present students with an ill-structured problem.
2. Direct them to develop a "tentative problem statement" from what they already know about the issue. Some information may be presented to them in the form of a Scenario Statement provided by the teacher or a supporting internet site.
3. Have the students create a list of what they need to know, in the form of questions. These questions provide direction for their research. Inevitably some questions will be answered, some later deemed irrelevant and new ones arise.
4. The closing phase of a PBL unit will entail making some recommendations nested in some form of professionally styled report.

Curricular Groups Who are Employing Problem Based Learning:

Earth System Science Community This links to the homepage of Earth System Science Community (ESSC) which defines "Earth System Science" as a holistic approach to the study of the Earth that stresses investigations of the interactions among the Earth's components (atmospheric, hydrologic, geologic, biological and biogeochemical). The greater purpose is to explain Earth dynamics, Earth evolution and global change and develop the capability to understand changes that will occur in the next decade to century, both naturally and in response to human activity. Their web site is comprised of a scheduled on-line curriculum with supporting data, images and powerful visualizers.