It is a popular belief that science is supposed to provide a literally or objectively true account of the world. Philosophers call this the thesis of scientific realism. Yet an inspection of the actual language used by scientists reveals that it is full of metaphor. Metaphor, it is frequently said, provides 'a lens through which we see the world.' What does this mean for the question of scientific realism? Is this lens objective or does it distort reality? By tracing the history of key metaphors in the development of cell theory and cell biology, and analyzing the valuable cognitive work they perform, this book argues that metaphors are, like microscopes and other instruments, a vital 'tool' in the construction of scientific knowledge and explanations of how the world works. More than just rhetorical devices for conveying difficult ideas to non-specialists, metaphors provide the conceptual means with which scientists interpret and intervene in the world. When properly understood the use of metaphors allow still for a viable form of scientific realism. Terms and phrases originally intended metaphorically may eventually come to be used quite literally to express what we call ‘objective scientific truths.’ Metaphors are powerful tools with which scientists ‘rewire,’ ‘reprogram,’ or ‘edit’ the cell’s genetic and biochemical ‘signaling circuits,’ to make them more efficient cell ‘factories’ or to ‘switch on’ a cancer cell’s ‘death program.’ They are a vital component in how humans achieve scientific understanding of the world and attempt to change and direct its processes.