Woody plant stress has many causes that might ultimately lead to plant decline. Tree and shrub degeneration is often referred to as a "complex," meaning the condition is usually caused by multiple factors. Typically, one or more primary stresses cause deterioration of plant health, followed by secondary pathogens and/or insects that further decline or destroy plants. Determining causes of decline requires careful examination of plants and growing sites, as well as knowledge of site history. Nevertheless, diagnoses may be difficult, as the original cause(s) of plant stress may be obscure or no longer present. Some of the most common plant stresses are addressed in this publication. A wider range of possible causes of plant stress and decline should be considered during evaluation of woody plant material.