The Plasmodium interspersed repeats (pirs) are described as the largest multigene family found in the genome non-Laveranian species of the malaria parasite, Plasmodium; however, their function remains unknown. In this thesis, I analyse pir transcription across the lifecycle of three Plasmodium species, using published and unpublished data, to identify potential targets for investigating pir function. RNAseq data from the rodent parasite Plasmodium berghei demonstrated that pir transcription peaks in the liver merosomes/merozoites, the asexual blood stages, and the male gametocytes. Detailed analysis of synchronous in vivo asexual blood stages of another rodent parasite, Plasmodium chabaudi chabaudi, showed that pir transcription peaks in the late trophozoites. Schizont stages of both rodent species demonstrate an enrichment for L-form pirs, linking this sub-family to schizogony. With de novo transcriptomics methods applied to the pirs of the human parasite Plasmodium vivax, I observed that transcription of pirs in the mosquito stages was minimal, in agreement with the data from P. berghei. The P. vivax asexual blood cycle pattern of pir transcription also showed high expression, but timings contrasted to that of P. c. chabaudi as transcription peaked in late schizonts/early rings. Overall, the results suggest that pirs are important during the lifecycle stages in the blood, and not in the insect vector. In all three Plasmodium species, I found the conserved ancestral pir to be highly transcribed, indicative of an essential function. In conclusion, by collating data from a range of sources I have identified consistent patterns of pir transcription between Plasmodium species at different stages of their lifecycle. Based on these results, I have postulated on the possible role of pirs in the biology of the parasite. This study will enable more targeted future studies into elucidating the function of this elusive multigene family.