Security should never be an afterthought in plugin development. Always sanitize user input using functions like sanitize_text_field() and validate data with isset() checks. Escape output with esc_html(), esc_attr(), and esc_js() depending on context. Use WordPress nonces for form submissions to prevent CSRF attacks. Never trust user input, always assume it’s malicious until verified. Implement proper capability checks before executing admin functions. Keep dependencies updated and follow the WordPress security handbook guidelines. Regular security audits of your code can prevent vulnerabilities before they become issues.