This dissertation examines complex theological-philosophical speculations concerning the concept of nonbeing, or meontology, in the classical period of Islam. The concept has been discussed in various settings such as (1) the doctrine of creatio ex nihilo, (2) the view of evil as privation of good, (3) the disontological account of God, and (4) a notion of a dialectical, epistemological, and semantic nature. Modern scholars have studied the reception of John Philoponus’ understanding of creatio ex nihilo in Islamic thought closely enough. On the other hand, the Neoplatonic view of evil as privation of good did not attract close attention of classical Muslim intellectuals. Therefore, this project particularly treats the last two dimensions of the concept. It accordingly consists of two parts of equal length. In Part I, I discuss the emergence of nonbeing as a notion of a dialectical, epistemological, and semantic character in the classical period. Chapter 1 explores the origin of this notion. We encounter preliminary considerations of the notion in the Imamite discussions of the second half of the 8th century on the doctrine of badā’ (God’s change of his decision), which in turn led to the formation of late-8th-first half 9th century Imamī-Mu‘tazilite controversy over the question whether God ab aeterno knows nonexistents. In Chapter 2, I reconsider the origin of the Avicennian essence-existence distinction by tracing it back to discussions of the nonexistent and the theory of aḥwāl (states). Chapter 3 is a continuation of the argument proposed in the previous chapter. I explore the philological and logical basis for Basran Mu‘tazilī-Ash‘arī theologians’ opposing approaches to the doctrine of ṣifat al-dhāt expressed in the form of self-identical sentences (S is S), e.g., “The substance is a substance in the state of non-existence.” I suggested that their approaches could be read against the backdrop of the two distinct interpretations of the Arabic copula verb kawn (to be) either as expressing identity/essence or existence. In Part II of the dissertation, I specifically deal with the meontological ideas of the 10th century Ismaili theologians of the Fatimid period, Muḥammad ibn Aḥmad al-Nasafī (d, 943), Abū Ya‘qūb al-Sijistānī (d. 971) and Ḥamīd al-Dīn al-Kirmānī (d. 1021). This part has three fundamental goals. First, I explore that Ismaili authors reconstruct their account of apophasis through frequent negation of me/ontological concepts. Therefore, I discuss in some detail the complexities of these concepts in their understanding. Second, it will be shown that these authors are not uniform in their perspectives. Finally, I investigate how their use of negation (nafy) as an expression of God’s transcendence builds a strong link connecting their point of view to the Neoplatonic tradition. In Chapters 4-6, I discuss, respectively, the failure of al-Nasafī’s attempt to form the negative path of speaking of God in line with double negation, al-Sijistānī’s role in the development of the Ismaili double negation in comply with the hierarchical cosmology of Neoplatonism, and al-Kirmānī’s deviation from double negation in his later work Kitāb al-Riyāḍ, though he remains loyal to it in Rāḥat al-‘Aql.