A bioindicator is an organism or biological response that reveals the presence of pollutants by the occurrence of typical symptoms or measurable responses and is, therefore, more qualitative.
These organisms (or communities of organisms) can be used to deliver information on alterations in the environment or the quantity of environmental pollutants by changing in one of the following ways: physiologically, chemically or behaviourally. 
The information can be deduced through the study of:
# their content of certain elements or compounds
# their morphological or cellular structure
# metabolic biochemical processes
# behaviour
# population structure(s).
The importance and relevance of biomonitors, rather than man-made equipment, are justified by the observation that the best indicator of the status of a species or system is itself. Bioindicators can reveal indirect biotic effects of pollutants when many physical or chemical measurements cannot. Through bioindicators, scientists need to observe only the single indicating species to check on the environment rather than monitor the whole community. Small sets of indicator species can also be used to predict species richness for multiple taxonomic groups.
The use of a biomonitor is described as biological monitoring and is the use of the properties of an organism to obtain information on certain aspects of the biosphere. Biomonitoring of air pollutants can be passive or active. Experts use passive methods to observe plants growing naturally within the area of interest. Active methods are used to detect the presence of air pollutants by placing test plants of known response and genotype into the study area.
The use of a biomonitor is described as biological monitoring. This refers to the measurement of specific properties of an organism to obtain information on the surrounding physical and chemical environment.
Bioaccumulative indicators are frequently regarded as biomonitors. Depending on the organism selected and their use, there are several types of bioindicators.