When oil-storing forms of phytoplankton die and descend to the bottom of the sea and undergo microbial degradation, they are often covered by huge sediment load and subjected to enonnous pressure. Such conditions are supposed to change the deposited phytoplankton into fossil fuel after hundreds and millions of years. Thus human world is benefited from these tiny organisms even after their death. It is therefore needed to know about these creatures in more details for their proper conservation, management and culture. This book is a humble endeavour to know about their cell structure, classification, uses and the various negative stresses operating in the marine and estuarine environments to disrupt their community.