Introduction We all want healthy forests. But what, exactly, is a "healthy" forest? Would we necessarily know one when we see it? The fact is, forests exist in various stages of physical reality, and whether or not we view different conditions as healthy is often based as much on our desires and interpretations as on any empirical evidence. So forest health is largely in the eye of the beholder, which makes for a difficult debate. How are we to agree on what constitutes a healthy forest when for all practical purposes none of us are looking at the same forest? (right) Mosaics of different tree species and ages provide habitat for a wide range of wildlife. The difficulty of defining forest health ensnares both professionals and the general public, since we all harbor feelings (often very strong ones) about what we want from them. To make things more confusing, forest conditions are complex and seldom permanent. Forests go through many stages as they become established, change through the growth and aging of the dominant trees, or are dramatically affected by a disturbance like fire, flooding, or windstorms. All of those conditions may, at one time or another, be part of the natural dynamics within a healthy forest. Or they may be a sign that something is terribly wrong. Knowing the difference may tax the best knowledge we possess. We also have trouble establishing a clear notion of how humans should interact with the forest. Again, it is a clash between different perceptions and desires. For example, there is an oft-expressed sentiment that forests should be left entirely alone, that "natural conditions" are best. This "let-nature-take-its-course" idea persists although it has been generally rejected by the scientific community as impractical and possibly destructive in some cases. Similarly, the status quo---that is, how our forests are currently being managed---may not be optimal either. Science increasingly views all ecosystems (including forests) as constantly in flux and always affected by human activities. In fact, much of human history is intertwined with the development of the forests. In short, we are as much a creature of the forest as the whitetail deer, the great-homed owl, and the dog tick. And, the forests that have developed over time have been shaped by the people in them, as well as all those other critters. Therefore, it is naive to say "get the people out of the forest!" That's probably not realistic---or good---for either the people or the forests. However, there are certainly biological and ecological changes taking place today that represent a very real deterioration of the forest environment. Therefore, it would be equally nearsighted and foolish to say "stay the course!" For our purposes, "forest health" means "a condition of forest ecosystems that sustains their complexity while providing for human needs." This definition encompasses the complex interactions of biological processes and human judgments that enter into discussions of the concept called forest health. The definition also recognizes, as former Forest Service Chief Jack Ward Thomas often points out, that a forest's health needs to be judged within the context of peoples' expectations for it. Not all places can---or should be expected to---produce the same mix of values, and if a forest's condition allows it to sustain itself and produce a mix of values deemed to be appropriate, most people would recognize that as a healthy condition. We also want to make it plain that healthy forests are produced by entire systems, not just trees. As Forest Service Chief Mike Dombeck recently reminded a Congressional committee: "A healthy forest is one that maintains the function, diversity, and resiliency of all its components, such as wildlife and fish habitat, riparian areas, soils, rangelands, and economic potential . . ." (Dombeck 1997). But trees---the most visible and dominant structures in a forest---are often the focus of attention, particularly when political debates break out about how to handle certain situations, a post-wildfire timber salvage, for example. While much of the controversy may focus on trees, much of the value of healthy forest ecosystems lies elsewhere. As ecologist Robert Costanza and others have pointed out, the services provided by all ecological systems (not just forests) are so critical to the functioning of the Earth's life-support system that if those systems failed, the world's economies would tumble. Forest ecosystems comprise one-third of America's land, and productive forests represent a major part of many regional economies, making forests that function in a healthy and sustainable condition critical to the continuation of a functioning society. It follows, therefore, that healthy forests ought to be sustainable, insofar as we can define that goal. One definition of sustainable forest management is "to meet the needs of the present without compromising the ability of future generations to meet their own needs" (AF&PA 1997). That borrows the concept of sustainable development introduced by the United Nations Commission on Sustainable Development, often called the "Brundtland Commission" after its chair, Norwegian Prime Minister Gro Harlem Brundtland. The idea of sustainability is completely consistent with the forest health definition, because any forest that cannot "sustain its complexity," cannot "meet the needs" of either present or future generations. We call what people do in relation to forests "management," whether it is intentional or not. In many discussions, management is a term for actions such as harvesting timber or planting trees. We believe, however, that many other human actions are of great importance in affecting forest condition and health. When people remove human influence from an area, suppress fires, introduce an exotic species, alter hunting or predation rates, or inject chemically active materials into the environment, forests are affected. Some of these actions may be an attempt to preserve a particular forest condition that people value, but most are taken with little or no thought of how forests will be affected. Often overlooked in the forest health debate is the importance of experience. Native Americans observed their forests for thousands of years, leaming to use management tools like fire to create the type of forest they desired. What those early Americans learned, they passed down from generation to generation (Cronin 1983; Pyne 1982). In contrast, modern American society has only had a few hundred years to interact with the forest. We've learned much, but we increasingly recognize the need to speed up the learning process with the best science and prediction models we can develop. We may not have the luxury of another 500 years to learn how to manage these complex systems sustainably. We will illustrate some of the concepts, tools, and management techniques currently being used to maintain healthy forests or restore those that are in deteriorated or hazardous conditions. As good as some of the current practices are, newer science and further experience will teach new lessons, and good managers will adapt their techniques when they see the need. As Americans we need to realize that, to some extent, we are all managers of the forest. When people recognize a forest condition that threatens to render that forest unsustainable, they are ethically bound to do their best to remedy that problem. In spite of all the disagreements over how to manage our forests, we can all agree that these are real places, with real living organisms, going through real changes. It may take longer than our lifetimes for the results of these changes to become apparent, but the forests themselves are ultimately going to teach us what is sustainable and what is not. Each generation, therefore, is responsible for using the best techniques it knows. Obviously, debate will continue on what techniques are "best" in any given situation. Therein---particularly in the case of the public forests---lies the exercise in discussion, debate, and democracy that envelops the question of forest health.