Most tardigrades are not much bigger than 100 micrometers.

Tardigrades, together with nematodes, rotifers, mites and a few more rarely encountered groups are among the smallest animals and they are smaller than many of the bigger among the unicellular eukaryotes. That is why they have been discovered only after the invention of the microscope.

The tardigrades have evolved towards smaller and smaller sizes very early, already during the Cambrian. It is interesting that they are segmented animals, like their relatives the arthropods and the velvet worms, but they have very few segments, because in order to achieve such a small size they have lost all intermediate segments, so the segments that now form their body were originally the segments of the head, and now they are followed immediately by the original segments of the tail, without the original body that connected the head to the tail. Thus they have been miniaturized by losing their body and becoming a walking head (the legs of the tardigrades are what in arthropods have become appendages of the mouth, e.g. mandibles and maxillae).