In this study the potential use of thermochromic colorants in swimwear garments was investigated and the effectiveness of a colour transition in and out of water as a result of changes in temperature assessed. This project considers the use of thermochromic colorants for end use application in Speedo brand swimsuits for women and the opportunity for producing multiple colour transitions or even changes in design pattern. Prior to the application of thermochromic colorants to swimwear, a number of factors must be considered, most importantly, the colorimetric properties and fastness properties of thermochromic colorants applied to the textiles. Currently there is limited information available relating to the colorimetric and fastness properties of thermochromic colorants, particularly where applied to textile substrates. A novel colour measurement methodology suitable for the dynamic colour measurement of thermochromic colorants applied to textiles was developed. Investigations showed that thermochromic colorants exhibited different rates of decolourisation and re-colourisation as well as varying colorimetric behaviour depending on the maximum exposure temperature. The developed colour measurement methodology was used to investigate the fastness properties of thermochromic colorants applied to a polyamide/elastane swimwear fabric. The test samples were found to have varying wash fastness properties depending on the colour of the thermochromic colorant. Furthermore, samples were found to have unsatisfactory fastness to a simulated sunlight (xenon arc light source). However, it was found that by removing specific wavelengths of visible light, using a filter, the rate of degradation was reduced. Rub fastness properties in the dry state were acceptable, though loss of the thermochromic colorants was observed in the wet state. Exposure to chlorinated water adversely affected the test samples causing yellowing, particularly in the thermochromic decolourised state. Thermography and pressure testing techniques were employed in order to identify the areas of Speedo swimsuits most suitable for the placement of thermochromic print motifs. Garment pressure test results showed that the shape of a swimsuit and the fabric from which it was made was found to impact the fit. Thermography testing identified that the fit of a swimsuit had an effect on the convection of heat from the body to the swimsuit. The test results also showed that temperature distribution varied depending on the distribution of muscle and fat on the body. Thermochromic colorants were screen printed on to digitally printed ground substrates to investigate the colour transition effects achievable. Due to the translucency of thermochromic colorants colour mixing with the colour of the ground substrate occurred in the thermochromic coloured state. Print motifs suitable for use in Speedo swimwear were developed and conceptual diagrams showing dynamic thermochromic placement prints for swimwear were created.