I'm adding AdSanse codes to php file and they appear on every page of my site, so I suggest you to do like I do.

There are also themes that already have ad spaces integrated, so you just have to insert your ad code in the correct field. Themes like this are for example Heatmap, CTR, etc.

You have to be careful with some of these adsense plugins out there. Many of them (even those in the wordpress directory) skim a little bit off your adsense.

It's really easy to add the codes directly into your Wordpress theme.

Your best bet is to search the plugin directory: WordPress › Search for adsense « WordPress Plugins

and view the ratings of the different plugins available. Hope that helps.

I use WordPress › Wp-Insert « WordPress Plugins (it can do a lot more than insert Adsense).

Another one I used in the past is WordPress › All in One Adsense and YPN Pro « WordPress Plugins (and its predecessor) - but I got worried about "skimming", like someone above mentioned, so I dropped it. Maybe it's OK to use now. (Skimming is where the plugin developer inserts his own Adsense ID in the ads a certain number of times, or percentage of times - so you don't get credit for those. Some will be transparent about this, some are not)

There are a lot adsense plugins available such as adinjection,quick adsense etc

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