Bullying in schools is one of the features of crisis in the contemporary school system. It threatens pupil’s security and quality of learning. Bullying as a disruptive behaviour has been described as a subset of aggressive behavior that involves an intention to hurt another person by a variety of means. It includes physical and verbal assault and social exclusion. The purpose of the study was to establish teachers’ and students perceptions of psycho-physiological causes of bullying among public Secondary school students in Western Province, Kenya. The study was based on Albert Bandura’s social learning theory and Kurt Lewin’s field theory of perception. A descriptive survey research design was adopted. The study population was composed of 6,354 teachers and 65,969 form two students. Stratified random sampling technique was used to select students from 213 secondary schools. Purposive sampling technique was used to select teachers. Questionnaires and in-depth interview guide was used to collect data from the respondents. A pilot study was carried out to establish the reliability and validity of the data collection instruments. Qualitative data was transcribed and reported according to emerging themes while quantitative data was analyzed using descriptive statistics such as the frequency counts, means and percentages. Inferential statistics such as the Kruskal- Wallis one- Way analysis of variance was applied. The perceived psycho-physiological causes of violent behavior were; being physically strong, having mental disturbances, and being older than the victims as third. The study recommends that: school induction programmes for newcomers be established; consistent disciplinary measures be enforced; and establishment and monitoring of anti-bullying policy in all schools be enhanced.