Sell investments at a loss to offset realized gains. Excess losses can offset $3,000 of ordinary income (US) with carryforward.
Wash Sale Rule
Do not repurchase substantially identical securities within 30 days before or after the sale.
Where
Most valuable in taxable accounts—avoid harvesting in IRAs incorrectly.
Conclusion
Many robo-advisors automate harvesting—understand the strategy before outsourcing.