This chapter deals primarily with the interrelationship (the ecology) of biota (life-forms) in placid water bodies (lakes) and running water (streams). There are two major subdivisions of ecology: autecology and synecology. Each division of ecology has its own set of terms that are essential for communication between ecologists and those who are studying running and standing water ecological systems. The chapter discusses the pathways of water flow contributing to the stream flow, namely, precipitation inputs as they contribute to the flow. Insect macroinvertebrates are ubiquitous in streams and are often represented by many species. Although the numbers refer to aquatic species, a majority is to be found in streams. Lotic insects are found among many different orders, and a brief account of their biology is presented. Riffle beetle adults are considered better indicators of water quality than larvae because they have been subjected to water quality conditions over a longer period.