Biology studies of function and the mechanism of function of the various components of plants and animals. Questions are asked of tissues at every level of structural complexity, ranging from questions of human behavior (how does a conscious human control the position of his joints?), to questions of the physics of ionic movement (how does an ion cross a membrane 3 nm thick?). Biological questions have a unity because the evolutionary process which built biological structures is the same, whether the structures are enzymes or ankles. Biological questions have a unity because the methods of describing and analyzing function are not diverse, even if the structures performing those functions are. For example, the mathematics describing the flow of current through a hole in a protein is closely related to the mathematics describing diffusion across membranes which in turn is closely related to the mathematics describing the control system for movement: a particular differential equation is that differential equation, no matter what the meaning of its coefficients. The unity of biology arises then from the questions it asks of biological systems and the unity of the mechanism that created those systems.The challenge of biology comes from its complexity, more than anything else. That complexity arises in part from the diversity of systems and length scales. The complexity arises in part, of course, from the engineering (i.e., structural and biochemical) complexity of the systems of life themselves. But the complexity also comes in part from our ignorance. Until systems are understood, we are doing reverse engineering—solving an inverse problem—trying to determine how something works from limited measurements of inputs, outputs, and structure. Until we know what measurements best help us to understand and control the system, we will measure to much and the system will seem much more complicated than it may be. Such aapparent complexity is characteristic of the inverse problems of reverse engineering and often disappears (or is dramatically reduced) when the‘design equations’ of the system are finally discovered. The excitement of biology arises from the tension between its unity and complexity. When new systems arise, old questions are asked of them, at first. When new questions or methods arise in the new system, they are then applied to old problems. And applied to old problems, the new questions are often as revealing as when applied to the new systems, where they arose. Whether the (apparent) unity of biology is a reflection of reality (that is of general properties of evolved systems), or a reflection of the paucity of our knowledge, or even a reflection of the inherent limitations of our own ability to understand (i.e., of our nervous system), is not known. Probably all three.