On several engineering applications high Reliability is one of the most wanted features. The aspects of Reliability play a key role in design projects of aircraft, spacecraft, automotive, medical, bank systems, and so, avoiding loss of life, property, or costly recalls. The highly reliable systems are designed to work continuously, even upon external threats and internal Failures. Very convenient is the fact that the term 'Failure' may have its meaning tailored to the context of interesting, as its general definition refers to it as "any deviation from the specified behavior of a system". The above-mentioned 'deviation' may refer to: performance degradation, operational misbehavior, deviation of environmental qualification levels, Safety hazards, etc. Nevertheless, Reliability is not the only requirement for a modern system. Other features as Availability, Integrity, Security and Safety are always part of the same technical specification, in a same level of importance.This paper intends to present the most important points to be considered for building a Fault-Tolerant System applied to the attitude control system of a spacecraft project. To accomplish its intent, the paper starts by a theoretical review of definitions and concepts. After that, the requirements of a spacecraft project will be selected and/or written. Then, two possible solutions for the requirements will be proposed, and then compared. As the conclusion of this work, the key requirements for specifying a Fault-Tolerant Systems will be highlighted. Besides, a comparison between the two proposed architectures will be performed. Finally, possible applications to the automotive system will be commented.