The design of a spacecraft requires performing of a series of trade-offs, either at component level, sub-system or system level. This paper aims to focus the attention of space engineers on the detailed definition and choice of temperature sensors for spacecraft applications. Although the use of previous experiences and lessons-learned should be taken into consideration, the “already flown before” philosophy (If it has flown before in design-X then it has to work in our design-Y!!) should be avoided if an “optimum” solution is to be reached. This should be achieved by correct system level requirement identification and interpretation (temperature ranges, accuracies, project schedules, etc.). The scope of this paper is limited to the analysis of requirements and trade-offs; the full definition of a sensor and its electronics support is not treated in this paper.