Your monitor is too low.
Placing the monitor too low can cause you to tilt your head forward and lead to headaches. Make sure it's positioned properly (usually so that the top of the screen is level with your eyes). See Monitor Setup and Usage.
Glare or reflection on your screen is causing eyestrain.
Glare can cause you to squint, forcing the muscles in your eyes to work harder than usual. Make sure to keep your screen clean, and free of glare or reflections. You may also want to consider an anti-glare screen for your monitor See Monitor Setup and Usage or Monitor Overview and Accessories.
Your monitor is too close.
Just like a TV or movie screen, sitting too close forces you to strain your eyes; doing it for too long can lead to headaches. Your screen should be about arm's length (24-30 inches) away. See Monitor Setup and Usage.
There's inadequate lighting or too much lighting.
Inadequate lighting can force your eyes to "work overtime" to read from the screen (or force you to move in closer so that it's visible). Too much lighting can cause glare or reflection. Both may lead to eyestrain and headaches. For a complete discussion of lighting issues, see Lighting Setup and Usage and See Monitor Setup and Usage.
You've been working too long without resting your eyes.
Your eyes are muscles, too, and need regular breaks to avoid feeling weary from overuse. Follow the 20/20/20 rule - every 20 minutes, look at least 20 feet away for 20 seconds. See Monitor Setup and Usage.
Your font size is too small.
If your text is too small, you may be squinting to see it - or leaning in towards your monitor, also causing back pain. You have a few options; you can adjust the size of your font, change your screen resolution, or consider a screen magnifier. See Monitor Setup and Usage or Monitor Overview and Accessories.
There's poor contrast on your screen, or the refresh rate is too low.
Working with inadequate contrast settings can force your eyes to work harder to differentiate characters. A refresh rate that's too low can cause both eyestrain and headaches. To learn how to properly adjust both, see Lighting Setup and Usage, See Monitor Setup and Usage, or the article Reducing Eyestrain.