Cirrus clouds are thought to have a significant role in atmospheric processes: specifically; their heating/cooling contribution to the Earth's radiative balance, and the consumption of water substance due to their formation. Their presence in the upper troposphere I lower stratosphere (UTLS) also provides a surface for heterogeneous chemistry. The SLIM CAT-Cirrus model is developed to provide a tool to investigate aspects of these properties. SLIM CAT-Cirrus is based upon the existing SLIM CAT chemistry transport model and a parameterisation of the formation of cirrus ice by homogeneous nucleation. The advantages and drawbacks of the use of legacy models are discussed especially issues regarding the loss of the underlying decision-making regarding design approach, approximations, and assumptions. Techniques adopted and adapted from the software engineering and QA disciplines are used to mitigate these problems and maintain future traceabilty; this takes the form of examples of practical measures that small groups or individuals researchers can use. The difficulty in validating a complex global model in the absence of a definitive reference has been addressed by using diverse measurement data sources, and a suite of statistical merries. Model verification testing is also used to characterise processes that are difficult [0 validate. Validation of the modelled frequency of cirrus occurrence against satellite data showed an initial under-prognosis by the model. To address this a statistical scheme has been devised to reproduce some of the effects of phenomena such as gravity waves that are not resolved by the model grid. The modelled effects of the formation of cirrus on the water budget in the UTLS are comparable with measurements from the HALOE (HALogen Occultation Experiment), and are also in-line with the drying effect cirrus are thought to have on air entering the stratosphere. The radiative effects of cirrus have been represented using specific cirrus radiative parameterisations. The cirrus heating shows positive feedback into vertical transport causing meso-scale uplift of the kind thought to be responsible for part of the BrewerDobson atmospheric circulation.