How liquid does an ETF need to be before I should worry about trading it?

Liquidity means how easily you can buy or sell without moving the price. For an ETF, look at two things: the fund's trading volume and the liquidity of what it holds. A huge S&P 500 ETF trading millions of shares daily has razor-thin spreads and you can trade any reasonable amount instantly. A small, niche ETF with low volume may show a wide bid-ask spread and can be costly to trade even if the underlying stocks are liquid, because an ETF's true liquidity also depends on its holdings, not just its own volume. To stay safe, favor larger, established funds; use limit orders on anything low-volume; avoid trading in the first and last few minutes of the day when spreads widen; and skip trading during market halts or extreme volatility.