As the knowledge of cervical carcinogenesis improved, it became imperative to unify the different terminologies and to effectively communicate with the clinicians so as to optimize the management of patients. Thus, a meeting was convened with experts from all over the world at National Institutes of Health in Bethesda, Maryland, USA, in 1988. As an outcome of this workshop, the TBS reporting for cervical cytology was introduced in 1991 to provide uniform system of terminology for reporting with clear guidelines for the management of these lesions.
The two-tiered system of low-grade SIL (LSIL) and high-grade SIL (HSIL) matches the HPV carcinogenic potential and allows for better communication between pathologists and other patient care providers.  The Bethesda System (TBS) for reporting cervical cytology follows this approach (updated in 2015) [Figure 1]. This system is also applicable with minor modifications for vaginal cytology and anal Pap.
Keratinizing HSIL cells showing pyknotic and hyperchromatic nuclei with high N: C ratio with coarse and irregularly distributed chromatin with attenuated cytoplasm. The numerous bizarre keratinizing (orange) cells and the granular background are highly suggestive of invasion. Image from The Bethesda System for reporting cervical cytology. 153554b96e