This paper aimed to shed light on the Classical Test Theory (CTT), which is one of the simplest and oldest theories in psychological and educational measurement. It traces CTT's development from the early 20th century by pioneers like Charles Spearman, Galton, Lord, and Toffler. Also, it aimed to clarify the basic concepts of the theory, like true score, error score, reliability, and validity, highlighting their role in test development. The paper also focused on practical applications of CTT in measuring human traits and abilities, emphasizing its influence on test score interpretation. The results that the article reached are: CTT distinguishes between observed score (X), true score (T), and error score (E). True score reflects actual ability, while error score represents random influences. Reliability is calculated as the ratio of true score variance to total variance. Despite the appearance of new theories like Item Response Theory (IRT), CTT remains widely used due to its simplicity and ease of application.