Nucleic acid aptamers which bind specifically to or are internalised into cells expressing a CD7 cell-surface receptor are disclosed. The aptamers comprise a nucleic acid molecule having a sequence which is at least 80% identical to 5′-GG-GAGACAAGAAUAAGCAUG-R1-UUCGACAGGAGGCUCACAACAGnynz-3′ (SEQ ID NO: 3). The aptamers can be linked to therapeutic or diagnostic molecules, and can be used to deliver the therapeutic or diagnostic molecules to cells expressing the CD7 receptor. The aptamers can also be used as detection tools.