Each refill kit contains ink for many refills, all the tools you will need, as well as detailed instructions in English. Black and colour refill kits are available separately and once you use up the supplied ink, individual colours can be bought individually (here) in different quantities. The supplied ink is not made by HP,  but is formulated specifically for HP printer ink cartridges and ink tanks. Premium quality, US made, quick drying dye or pigment ink. NOT universal or Chinese inks.

Can my HP ink cartridges be refilled? Any tricks?

1 - Refill HP cartridges before empty...
Most Hewlett Packard ink cartridges (with print-head) are pretty easy to refill, if you know how. HP cartridges should never be allowed to run out of ink. Refilling cartridges before or as soon as any colour (or black) starts to fade, can considerably extend the ink cartridge/print-head life. Topping the cartridges up frequently is best approach, but be careful not to overfill...

3 - Read (and follow) the refilling instructions...
While many HP cartridges may look similar, ink volumes and the refill process can be different from one cartridge to the next.   

4 - Invest in high capacity (XL) cartridges or ink tanks...
Many recent HP cartridges are available in either standard or high yield version. In the past, the only difference between high yield and standard cartridges was the amount of ink they came with. These days, not only do the standard cartridges come filled with less ink, but more often than not, the internal sponge that holds the ink is lower capacity as well. In other words, it may not be possible to fill the non-XL cartridges to full capacity.

Ink levels and HP...

Refill kits for HP # 564 and HP #920 now available...

HP original # 564 and 564XL cartridges as used in Photosmart A109, A209, A309, B8550, C5380, C6375, C6380, D5460 etc are fairly easy and very economical to refill. The ink chips fitted to these cartridges can not be reset, so you will loose the ink level indicators, but as these printers continue to function with "empty" cartridges, it's not a major problem. As the 564 ink tanks are not transparent, working out when to refill can be difficult without removing and inspecting the cartridges through the "window" on the bottom.

NEW INFO: HP genuine 564 cartridges have changed! Recent 564's don't have the ink inspection window on the bottom and more importantly - the internal design has been modified to make refilling harder. HP have removed the internal connection between the front and rear chamber in the new cartridges, so the cartridges can only be filled to 1/2 capacity (same as new). The only way to get around this is to buy high capacity (XL) cartridges. These can be filled to full capacity.    

Note: There are 2 black refill kits available for the "564" printers. H-564B is for printers with one black cartridge (eg Photosmart A209) and H-564BB for printers with two black cartridges (eg Photosmart A309). The colour refill kit H-564C refills the 3 colours (C/M/Y) and is same for all 564 cart printers.

HP920 cartridges are identical to HP564XL, use the same ink set / refill kits and refill exactly the same.

Tip: Save the original cartridge outlet (orange plastic) clips that come with new individual cartridges. You need one for the thin cartridges and one for the fat (564 black) if you want to refill.

Most original HP individual ink tanks (one for each colour) # 02, 10, 11, 12, 13, 14, 18, 82, 88, 940 etc are not easy to refill. If you want to refill any printer using these cartridges, take a look at refill friendly cartridges. They are really easy to fill and fitted with auto-reset ink level chips to overcome ink monitor issues.