Due diligence is the critical research and investigation period that occurs after a purchase agreement is signed. During this phase, investors verify the property's physical condition through inspections, confirm financial performance by reviewing rent rolls and expense histories, evaluate title for any liens or encumbrances, review zoning and environmental reports, and assess the local market. The due diligence period is typically 10-30 days and is often the last opportunity to discover deal-breaking issues before committing. Thorough due diligence is the investor's primary defense against overpaying, inheriting problems, or misunderstanding a property's true potential.