A direct investment by an LP alongside a GP into a specific portfolio-company transaction, typically on reduced or zero management fee and zero carry — a significant source of net-return uplift for LPs willing to execute the underwriting work on a deal-by-deal basis. Standard PE allocation increasingly mixes ~75% primary fund commitments, ~15% secondaries (LP-led and GP-led), and ~10-15% co-investments. Co-invest rights are negotiated in the LPA or side letter; large/strategic LPs (CalPERS, Harvard, GIC) command priority allocation, while smaller LPs receive co-invest opportunistically. Co-invest underwriting decisions are time-pressured (typically 2-4 weeks from offer to commitment).