I explore the concepts of existence and substance in the present paper. "To be" is the pivotal concept of ontology. Aristotle says that "there is a science which investigates being as being and the attributes which belong to this in virtue of its won nature." Descartes treats being as a perfection, but Kant clams that "to be" is not a real predicate. In the practice of modern logic "to be" is distinguished between the following three senses: identity, predication, and existence. In the second section, I discuss the various senses of Aristotle`s uses of "things are said to be". All of these uses can be presented in the formula of quantificational logic. In the remaining part, I discuss Ariatotle`s concept of substance. In particular, the distinction between species, genus, and accidental properties are clarified, the general structural features of substance are explicated.