As a group, geologists are engaged in the perhaps the morbid (though awe-inspiring) study of the past. It is therefore a pleasure to add a little of the present to this symposium, for it is in that sense that the word modern has been used in the title. We shall have a glimpse at the goings-on within a lake not so much in terms of factual material as in terms of principles and generalities. In order to do this I will discuss the lake in terms of simple physics and the over-all metabolism. When I am through you will probably find yourself no better equipped to undertake the study of a lake than you are now, though you may have a general understanding of the lake in urn to the study of interactions of biological and chemical factors (metabolism).