Effective dependency management and security scanning are crucial for secure development, as unmanaged dependencies can introduce vulnerabilities, malware, or malicious components into codebases, leading to security issues like vulnerability exploitation, malware injection, and license compliance problems. Best practices include keeping dependencies up-to-date, using trusted sources, monitoring dependency health, integrating security scanning into CI/CD pipelines, prioritizing and remediating vulnerabilities, and educating teams on secure development practices.