The primary objective of the present chapter is to examine the theoretical foundations of developmental dyslexia, discuss its etymology, and contrast the general definitions of dyslexia. This will facilitate comprehension of the concept of developmental dyslexia and investigation of the underlying factors. Furthermore, this chapter presents typologies of developmental dyslexia, followed by a discussion of dyslexia symptoms in younger children and older individuals. Subsequently, this chapter investigates developmental dyslexia and its diagnosis, with an emphasis on the role of IQ tests as dyslexia assessment instruments. Finally, the assessment of processes related to reading and writing difficulties is discussed.