Once you figure out the cause of dark circles, they become a little easier to effectively treat. The easiest way to do this is press on them gently. If they lighten up for a second , they are probably caused by little blood vessels showing up under the skin, where skin under the eye is thinner). If they don't, then it might be excess pigment, caused by sun exposure or even be hereditary.
THE QUICK FIX A under concealer should have good coverage, be easy to blend, and most importantly not crease. Always start with an eye cream or a under eye primer and let that soak in for at least a minute before applying concealer. Im always on the lookout for amazing formulas, heres my faves right now that have great coverage, contain good skincare ingredients, and have light diffusing properties :
smashbox photo finish under eye primer: has a little sheer luminous tint, perfect for prepping the under eyes before concealer
cle de peau concealer: this twist up stick formula is so richly pigmented, it could last years. but please don't keep it that long.
clinique airbrush concealer: this is my i don't have time to put on concealer concealer, because its super sheer & comes in a click pen style applicator, no separate brush needed!
nars radiant creamy concealer: creamy and hydrating, tons of shade options.
THE LONG TERM With most things in life, I always recommend trying the easy way first. For little blood vessels, try a cream at night that contains a retinoid. Retinoids are proven to build collagen and thicken the skin, which can help make the veins and blood vessels less visible. Start out with an over the counter version like Dr Dennis Gross Ferulic + Retinol Triple Eye Correction Serum. Once that runs out, talk to your dermatologist about a prescription retinoid to use at night. Tretinoin cream in a .025% strength seems to be well tolerated around the eyes. If you suffer from seasonal allergies, you can also try an over the counter antihistamine which can really help lessen the appearance of blood vessels. If you don't get enough improvement, a little medical intervention can help. Your dermatologist can do a few tiny injections of a hyaluronic acid filler which would sit closer to the surface of the skin and help cover the blood vessels.
If your dealing with under eye circles that are pigment related (and not blood vessels), the easiest option is to lighten them with a brightening cream. Some ingredients to look for are licorice, kojic acid, and hydroqunione. My top picks are Kate Sommerville Spot Reducing Concetrate or Philosophy Miracle Worker Dark Spot Corrector. These usually take about 4 weeks before you'll start seeing results. For more dramatic improvement, you can try a few laser treatments to remove the pigment. A few Clear and Brilliant or Fraxel treatments will erase any darkness over 3 to 5 treatments with little downtime.