Do electrons and genes exist? If inclined to answer 'yes', let's ask a harder question: do numbers exist? This book argues that the answer should be, again, affirmative. It elaborates a philosophical position according to which all, and only, entities truly indispensable to the formulation of modern scientific theories should be recognized as existent, regardless of how we might be initially tempted to categorize them - as concrete-physical or abstract-mathematical. In addition to explicating the subtleties of the positive reasons supporting this form of realism, the book clarifies and rebuts a variety of objections raised against this position.