It all depends on the reason for the build up.
Assuming you have traditional double-pane glass in your windows, there are a few things to look for if you notice moisture.
Often, moisture at the bottom of the windows is simply caused by too much humidity in your indoor air. If that’s the case, simply adjust your humidifier.
If the moisture is on the exterior of the window, typically there’s also no problem with the window itself. It may have rained recently or the outside humidity may have spiked causing the accumulation. Generally, there’s no reason for concern.
However, if the moisture is in between the two panes of glass, the seal has broken and surrounding air – along with its water content – has made its way in. This disrupts the thermal barrier of the window, reducing its energy efficiency. In fact, the glass might feel noticeably colder than your other windows on chilly days. In that case, you’ll need to replace the pane.
Similarly, if the moisture is coming in through only one spot — the bottom right corner, for example — then you might have a leak. If you have a wood frame or sill, you may also notice a growing water stain. It’s important to get leaks fixed quickly. There may be water damage occurring within the frame that you cannot see.