Bioindicators are organisms that can be used to monitor environmental health. Different types of bioindicators like plants, animals, and microbes indicate different types of pollution or environmental changes. Scientists observe changes in bioindicator populations to assess environmental conditions. This document provides examples of various bioindicator species and how they are used, including lichens for air quality, earthworms for soil toxicity, and diatoms for water acidity. It also outlines classifications of bioindicators and criteria for selecting effective bioindicator species.