Essential oils have become a hot commodity over the last few years. Whether you’re using them for aromatherapy or you just want your living room or office to smell great, you’ll need an oil diffuser to release beautiful scents into your environment.

There are a few different types of diffusers, but ultrasonic diffusers are the most common variety that you’ll find online for personal use. (All four of our top diffuser choices are ultrasonic diffusers). They plug into your wall, but they don’t use heat the same way that other diffusers do.

Instead, they use ultrasonic vibrations to push your oils to the top of the water, and then they vaporize the water and oils. The vapor releases your essential oils into the air, and it also acts as a mild humidifier. Your essential oils will last longer in ultrasonic diffusers because they’re being mixed with water.

The amount of water your diffuser holds determines how long it can continuously run. The ZAQ Dew Essential Oil Diffuser holds 30 milliliters of water and can run for up to four hours on one fill. The ASAKUKI Premium Essential Oil Diffuser has a 500-milliliter capacity, and it can run for up to 16 hours.

Many diffusers also come with soothing LED lights that change color as they run. URPOWER’s diffuser has seven different colors to choose from, while the ZAQ Dew rotates between five fun hues.

You’ll need to plug your ultrasonic diffuser into the wall to turn it on. Since oil diffusers are connected to an outlet, they have different timer options for safer use. The VicTsing Cool Mist Essential Oil Diffuser has four time-setting modes (one hour, three hours, six hours and continuous). The URPOWER diffuser has timer options for 60 minutes, 180 minutes, 360 minutes or continuous use.

There are hundreds of soothing diffusers to choose from on the web, but our picks have been thoroughly vetted. Check out our tips and advice below to figure out which one is the best for you.