A car seat is one purchase where the stakes are genuinely high enough that it's worth getting the basics right before you shop by price or looks alone.
Rear-facing vs. forward-facing
Current guidance from the American Academy of Pediatrics is to keep a child rear-facing as long as possible, up to the maximum weight or height limit set by that specific seat's manufacturer — not a fixed age. This replaced the older "switch at age 2" rule back in 2018; the AAP found there wasn't a clean age cutoff where the safety benefit of rear-facing stopped, so the guidance became "as long as the seat allows," which for many convertible seats today means well past a second birthday.
Once a child outgrows the rear-facing limit, the next stage is a forward-facing seat with a harness, again used up to that seat's own weight or height limit, before eventually moving to a booster seat that positions the vehicle's own seat belt correctly.
The two things that matter more than the model
Never put a rear-facing seat in a front seat with an active passenger airbag. In a crash, an airbag deploying against the back of a rear-facing seat can cause serious injury. Rear-facing seats belong in the back seat.
Check the seat's certification and expiration. Car seats are tested and certified to safety standards that vary by region — for example FMVSS 213 in the US or R129/i-Size in the EU — and they have an expiration date, typically several years from manufacture, after which the plastic and materials are no longer considered reliably crash-tested. That date is usually stamped or printed somewhere on the seat itself.
What to never buy secondhand
Avoid buying a used car seat unless you know its full history and are certain it's never been in a crash — even a crash that looks minor can compromise a seat's structural integrity in ways that aren't visible from the outside. If you don't have that certainty, buying new is the safer call for this specific item, even if you're comfortable buying secondhand for almost everything else.
The one thing people forget
Have the installation checked by a certified technician if at all possible — many fire departments, hospitals, and police stations offer free car seat inspections. Studies have repeatedly found that a large share of car seats are installed incorrectly by well-meaning parents, and a seat that's technically the right model doesn't protect a child properly if it's not installed correctly.