Why are my leaves turning brown?

Short answer: ...it depends!

Long answer: There are a few possible reasons...

.. and the culprit can usually be figured out by looking at the patterning of browning, along with how you've been taking care of the plant!

Check for pests

The first step, whenever a plant starts changing unexpectedly for the worse, is to look for pests. This should already be a regular part of your routine, but do it again, paying extra-close attention this time! Look for spider mite webbing in the nooks and crannies; white mealybug residue where the petioles meet the stems; telltale black dots or honeydew. If there are pests, they may very well be the cause of the browning! Take care of them first, and see if the situation improves or continues to deteriorate.

Brown tips

Brown tips usually means one of two things: either your humidity is too low, or you're overfertilizing!

Brown circle with a yellow ring

This usually indicates a fungal infection, and should be treated immediately!

Brown edges

This can mean that the humidity is too low, or it could mean that you're underwatering your plant, particularly if it's a thirsty species!

When in doubt -- go over your parameters! Make sure your humidity is in the right zone, that you're neither over- nor under-watering, and that you haven't been adding too much fertilizer.