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Over recent years the aquaculture industry has grown at a faster rate than any other major animal food production sector. This growth in the size of the sector has created a need for research into the fundamental biology of the animals being farmed. This PhD examined the relationship between the immune response and growth/development using several different model systems. Chapter 3 investigated effects of an acute pro-inflammatory stimulation on the transcriptomic output of Atlantic salmon primary muscle cells in vitro. In response to immune stimulation with the pro-inflammatory cytokine IL-1β there was an increase in the expression of several pathways including: the acute phase response, muscle catabolism, growth and cell cycle regulation. These results provide some evidence that the molecular basis of immune related growth reduction may be the same in fish as in higher vertebrates. Chapter 4 investigated the effect of the parr/smolt transformation (PST), a developmental stage during which salmon exhibit reduced disease resistance, on the expression of immune related genes in response to acute immune stimulation. This involved the use of the bacterial mimic (LPS) and the viral mimic (poly I:C), to elicit an immune response from salmon at different stages over the PST. The magnitude of response to immune stimulation was suppressed in multiple tissues over the PST. Chapter 5 investigated the ability of Rainbow trout fry at both first feeding and three weeks post first feeding to respond to immune challenge. The immune response increased in magnitude and complexity with age. The effects of growth rate on immune gene expression were investigated; there was a slight increase in the up-regulation of expression in immune related genes and protein degradation genes in the slow growing fish. The findings in this thesis provide further understanding of the interaction between growth/development and the immune response.