Suhrawardi was a Persian philosopher of the 12th century and the founder of the school of "illumination" (usdishr\bar aqusd). He not only critically analyzed the rationalistic philosophy of the Peripatetics but by drawing from a variety of traditions such as Zoroastrianism, Pythagoreans, Hermeticism and Neo-Platonic philosophy he created an entirely new philosophical paradigm whose influence is still strong in many parts of the Islamic world. The central task of my thesis was to undertake an indepth and detailed study of Suhrawardi's theory of knowledge. The question I was concerned with is what does Suhrawardi mean when he says he knows the truth? To analyze Suhrawardi's epistemology I have first considered most of his writings in Persian and Arabic where his theory of knowledge is discussed. I have then considered his critique of empiricism, rationalism and other means of gaining knowledge. Following a philosophical analysis of Suhrawardis ideas I have elaborated on his mystical writings and then attempted to demonstrate the following: (1) The means by which illumination is attained. (2) The rational basis of illumination as a pure methodology for the knowing of truth. (3) The scope and limits of this type of knowledge. (4) The nature of the type of knowledge that is attained through illumination. (5) The relationship between illumination, knowledge, truth and certainty. (6) Suhrawardi's theory of knowledge known as "knowledge as presence". Following an extensive discussion of the above topics I have considered the influence of Suhrawardi and his school of illumination on Islamic philosophy in Iran and other regions. This discussion which is historical in nature also explains the roots of some of the philosophical movements which were inspired by Suhrawardi Finally, I have offered a critical analysis of Suhrawardi's philosophy and some of the charges which have been made in regards to his philosophy.