This document presents the definition of SDG Thematic Indicator 4.a.2 to measure “safe, non-violent, inclusive and effective learning environments”: “Percentage of students who experienced bullying in the past 12 months”. This indicator was endorsed in January 2018 by the Technical Cooperation Group (TCG), which is the body in charge of the overall process for selecting SDG 4 indicators. The data presented in this document was updated in July 2021. Annex 1 describes the rationale and process for developing the definition of this indicator. Annex 2 provides the main concepts related to bullying according to international surveys. Annex 3 maps countries who conducted one or several surveys providing data for the indicator (before the definition of the indicator was finalized). Annex 4 includes all questions related to bullying in international surveys.