Reconnaissance and vulnerability assessment constitute the fundamental stages of penetration testing and cyber-attack lifecycle analysis. This chapter provides a systematic analysis of passive reconnaissance, active reconnaissance, and vulnerability assessment methodologies employed to detect, analyze, and assess security vulnerabilities in target systems and networks. It elucidates the methods by which threat actors gather and correlate publicly accessible information, engage directly with targets via scanning and enumeration, and utilize vulnerability scanning technologies to evaluate exposure. The chapter additionally examines the analysis of vulnerability scan results, the conversion of raw data into practical security insights, and prevalent obstacles faced during reconnaissance and assessment endeavors. Through the ethical replication of adversarial reconnaissance tactics, penetration testers can proactively find vulnerabilities, enhance defensive measures, and facilitate informed security decision-making.