Nowadays, most cities deal with the problem of “Urban Heat Islands”. Especially existing city districts cannot easily be adapted. In this paper, the effects of blue-green infrastructure elements (BGI) on air and surface temperature in courtyards are examined, based on on-site measurements and simulations. Recognizable effects on the temperature were observed: BGI lower the number of hot days in the courtyard, including a faster air temperature drop at night, but water elements increase the number of tropical nights due to their heat capacity. Model simulations with PALM-4U proved to be useful to analyze the effects of BGI on the microclimate. Besides analyzing existing structures, the effects of planned measures can be quantified by simulation. However, for this application, needs of improvement were recognized to evaluate the influence of BGI on the microclimate more realistically. For decision support, standard indicators such as the number of tropical nights and hot days are not differentiated enough to quantify specific climate stress of urban residents. It is suggested to consider summer days additionally, percentiles could be used instead of fixed thresholds and the entire course of the year should play a role in the evaluation of the elements and urban design.