The main aim of this thesis was to characterize mammalian chitotriosidase in terms of its function in innate immunity. This was achieved using several different approaches. In terms of gene expression, it was demonstrated that chitotriosidase is highly expressed in human peripheral blood monocyte-derived macrophages and can be induced further by β-glucan-containing PAMPs. This suggests that the enzyme may be involved specifically in anti-fungal immunity. Furthermore, chitotriosidase activity was detected in neutrophil pellets and the supernatants of cells treated with prolactin (consistent with its putative role in immune modulation) and the degranulating compound fMLP. The presence of chitotriosidase activity was also demonstrated in human tears; previously it has been shown that there are high levels of expression in the lachrymal glands of the eye, that also express lysozyme. As lysozyme has been shown to have antimicrobial activity in combination with other compounds and has additional chitinase activity, it was postulated that these enzymes may produce synergistic effects. The addition of recombinant human chitotriosidase, however, had no additional effect against Gram-positive or Gram-negative bacteria, when added with sub-optimal doses of lysozyme. It has been suggested that the loss of the mutant chitotriosidase genotype in two meso-endemic West African countries demonstrates that the enzyme plays a critical role in malaria prognosis. Furthermore, the homozygous mutant genotype has been linked with susceptibility to filarial infection. However in the current study of Papua New Guinean populations, it was shown that there is no correlation between genotype and the extent of hookworm infection. Furthermore, the frequency of the mutant allele was maintained at a level similar to those in non meso-endemic countries, suggesting that the enzyme does not play a critical role in the outcome of malaria. Phylogenetic analyses of the mammalian chitinase/chi-lectin gene cluster demonstrated that these genes have a highly conserved gene signature and may have evolved from a common AMCase ancestor gene. This would be consistent with the requirement for a chitin-digesting enzyme in primitive mammals. Through duplication and mutations events, this gene most likely produced subsets of other chitinases and chi-lectins, the latter evolving non-enzymatic functions, courtesy of the conserved α/β TIM-barrel domain.