Abstract: After a century of great strides in identifying the components and mechanisms out of which living things are constructed, biologists are increasingly turning (or returning) to questions of biological organization:  Why are biological systems built the way they are?  What explains the presence of the detailed mechanisms and patterns we observe?  How much is chance, and how much is necessity?  A defining feature of the recent Systems Biology movement is the tendency to explain biological organization in terms of design principles, i.e. strategies for achieving ends dictated by natural selection.  Following this approach I will discuss two examples of how selection for the ability to perform diverse tasks imposes constraints on biology, constraints that justify the need for complex patterns of organization that might otherwise seem arbitrary.  In one case I will talk about systems that implement robust pattern formation during animal development. In the other I will talk about stem cell systems, which underlie the development and/or maintenance of most tissues and organs, and which provide the context within which cancers arise.  In both cases, I will argue that approaching biology as a set of solutions to control problems provides more satisfying answers than are obtained by treating it as merely a complicated example of physics.