How Do You Find Your First Subcontracting Opportunity?

Prime contracts get the glory; subcontracting is how most small businesses actually break in. Where to look: SAM.gov subcontracting opportunities and SBA SUB-Net; primes' supplier diversity pages; agency OSDBU events; industry days (talk to primes' capture teams, not just the agency). How did you land your first subcontract? What would you do differently?