Security is paramount in plugin development. Always sanitize input using functions like sanitize_text_field and sanitize_email. Validate data before processing to prevent malicious submissions. Escape output with esc_html, esc_url, and esc_attr. Use nonces to verify request authenticity and prevent CSRF attacks. Check user capabilities with current_user_can before sensitive operations. Prepare database queries properly using wpdb prepare method. Never trust user input, even from administrators. Regular security audits and updates protect your users. Following WordPress coding standards enhances security by default.