5'-nucleotidases are proteins involved in nucleotide metabolism by dephosphorylating nucleotide monophosphates. A group of four related proteins (NT5DC1-4) has been described and an increasing amount of corresponding data has been published over the last years. Here, we review the current scientific literature on NT5DC proteins, present data on the four proteins, and discuss their potential involvement in cancer and other diseases. It seems that these proteins can have a role in various brain-related diseases, and there is a compelling amount of results showing that in particular NT5DC2 can be considered a drug target in cancer. More work is needed to conclude whether these proteins are involved or not in the nucleotide metabolism and thus potentially in purinergic signaling.