One of the best-known hair serums started with a bad color job. Moroccanoil co-founder Carmen Tal owned a small salon in Montreal when her hair was badly damaged, and on a trip to Israel a stylist used an argan oil treatment on it. She took two bottles home, convinced her husband, Ofer Tal, to get North American distribution, and that product became Moroccanoil Treatment, as she told Salon Magazine. It was sold in dark amber glass, which the brand says protects its UV-sensitive formula.

Today the words hair serum cover a lot. Some are silky oils for shine. Some are leave-in treatments that repair damage with proteins, amino acids or bond builders. Some work overnight. So the best hair serum is the one that matches the problem you actually have, not the one with the most reviews.

Many of the serums here also protect against heat, often up to 450°F, which means they can double as the last step before a blow-dry. That is the easiest way to make one earn its place on your shelf.

The first grid is hair serums and oils for shine and repair. The second is leave-in serums, milks and oils, several of them from newer brands. My tip: warm two or three drops between your palms, press them into your ends, then smooth what is left over the top layer.

Moroccanoil Treatment is still the reference point, with argan oil and linseed extract, and the brand says it boosts shine by up to 118%. Moroccanoil Mending Infusion is its split-end sibling, a light serum with argan oil, baobab and quinoa that seals ends for a fresh-cut look.

Gisou Honey Infused Hair Oil is the most reviewed serum in this edit, made with Mirsalehi honey to hydrate, add shine and smooth frizz. K18 Molecular Repair Hair Oil is the most technical, with the brand's K18PEPTIDE to repair the damage behind frizz, plus squalane and avocado oil.

Kérastase Elixir Ultime is the refillable classic, with wild camellia oil and heat protection up to 450°F. Kérastase Nutritive 8H Magic Night Serum works while you sleep, with niacinamide and plant proteins, and it protects hair from pillow friction.

Pureology Strength Cure Dream Healer is made for damaged and color-treated hair, with amino, citric and lactic acids that the brand says restore strength and seal split ends. Virtue Damage Reverse Hair Serum uses Alpha Keratin 60ku, a hair-identical protein, to refill lost keratin, and it is the one I would pick for hair that feels straw-like after bleach.

What I would skip: an oil as your only repair step on bleached hair. Pair a repair serum like K18 or Virtue with an oil on top, not instead.

This grid is lighter and a little more playful, and it includes a few formats that did not exist when hair serum meant one silky bottle.

Oribe Hair Alchemy Serum builds a protective layer around each strand with a watermelon, lychee and edelweiss complex plus hyaluronic acid, to help prevent breakage. Leonor Greyl Serum de Soie uses silk proteins and plant oils to detangle and smooth, and helps protect hair from sun and heat styling. R+Co Bleu F-Layer is a first step after washing that replenishes the hair's lipid layer with avocado, sunflower and coconut oils.

Virtue Healing Oil is a clear, shake-to-activate oil with Alpha Keratin 60ku that adds shine without weighing hair down. Crown Affair The Overnight Repair Serum is clinically shown to restore strength, softness and shine while you sleep, according to the brand.

Olaplex °7 Shine Serum is a two-in-one oil and repair mist in a super-fine spray, which makes it easy to use on hair that is already styled. Emi Jay Heavenly Hair Milk is a vanilla leave-in that detangles, cuts drying time and protects against heat. Pureology Hydrate Glow Catcher is made for dry, color-treated hair and claims 158 percent more shine.

A pairing tip: a milk or F-Layer on damp hair, then an oil on dry ends, covers most hair.

What I would skip: serum at the roots. Keep these on mid-lengths and ends unless the label says scalp.

I think the best hair serum is the one that fixes your specific problem. Choose an argan or honey oil for shine, a bond builder or keratin serum for damage, an overnight serum for dry ends, and a milk if you mostly air-dry. Use a few drops, on the ends first. The amber bottle from that salon visit started a whole shelf, and most of what sits on it now is better at one job than at all of them.