By default Android will scan your phones memory for any audio, video and pictures and add them to the media player/gallery by default. There are many reasons why this could be annoying. First there might be applications that use audio files (say a gps program). If the developer was inexperienced and/or lazy (and lets face it – most mobile developers at the moment are) no steps were taken to hide these files from the media scanner and they will show up in your music player. You might also have pictures/videos that you would not like to show up in the gallery. There exists a simple mechanism in Android to prevent this and it’s very similar to the way the robots.txt file works to control search engine spidering on websites (you can see where Google got the idea 
).
Simply create a blank file and call it .nomedia. Whatever directory this file is placed into will indicated to the Android media scanner that any media files inside this directory should not be indexed and left alone.
You can do this on the phone via Astro (or a similar file manager) or a terminal app (just do ‘touch .nomedia‘ – touch is a unix command that will create the specified file, but not actually populate it with any content).
You can also do this via your computer:
Windows - notepad, save as [all file types] .nomedia;
Linux/Mac et al – Use any file editor, file manager or the ‘touch’ command via the terminal).
If you are a developer – please be responsible and ensure that this file exists in any asset containing folder for your project 
Nokia – Please learn from this example since this was one of the most irritating parts of my whole Nokia experience 
This trick is simply amazing… Totally appreciate the author for putting it on web. Loved it.
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It works!!, thanks. Simply, Fast and Easy
How do we create a ‘file’? Only option available is create new folder.
There is an easy way to hide files. First you need to install ES file explorer which is free app an d using the same browse the file you need to hide. Only thing you need to do is rename the file and simply put dot(.) before the file name. Done.!! This file will be hide from galleries and media payers and there is also an option to sow hidden file in ES file explorer. Hope this will helps someone.
This may break other applications using those directories. For example a TV Guide app storing channel logos but not wanting to show these logos in Gallery. You can just create the .nomedia file with ES Explorer without breaking other functionnality.
Doesn’t seem to work on the Asus Padfone
SIMPLEST SOLUTION:
Just add a ‘dot’ (.) in the very beginning of the folder-name. (myfolder -> .myfolder) It works fine on my Galaxy Note. Excludes all mediafiles in folder plus subfolders from being indexed in “Gallery”, “videos” etc…
Looks like the .nomedia file covers all subdirectories too.
How do I get the effect OTHER THAN rebooting?
How can I add one to each and EVERY subdirectory on my sdcard? Manually is not an option that interests me. What lines would need be entered in a (b)ash script?
Useful, thanks
@erin
Enable your file manager to show hidden files, then delete the .nomedia file. The . in front of the filename tells the OS that this is a hidden file.
How do you reverse the .nomedia effect? Id like to remove it, an email would be helpful
To create .nomedia files in bash/ash:
cd to the directory
find . -type d -exec touch {}\.nomedia \;
To reverse:
find . -name .nomedia -exec rm {} \;
Why?? Why would you do it in such a complicated way?? The first command doesnt even make sence – your already in the directory and you run a find command for a directory just to execute a touch command – the first part doesnt even do anything.
cd directory
touch .nomedia
rm .nomedia
Why confuse inexperienced users??
Unfortunately some inexperienced users can’t even be bothered to read but confuse others by making false statements…
A previous poster asked how to put no media commands in all directories on the SD card – this is what that command does – It descends into a tree of directories and puts nomedia files in all of them – exactly what a previous poster asked for.