Abstract: Laser-assisted weapons, such as laser-guided bombs, laser-guided missiles and laser beamriding missiles pose a significant threat to military assets in the modern battlefield. Laser beamriding missiles are particularly hard to detect because they use low power lasers. They are even harder to defeat because current countermeasures are not designed to work against this threat [1]. The aim of this project is to examine the vulnerability of laser warning systems to guided weapons, to build an evaluation tool for laser warning receivers (LWRs) and seekers, and try to find suitable countermeasures for laser beamriding missiles that use low power lasers in their guidance systems. The project comes about because of the unexpected results obtained from extensive field trails carried out on various LWRs in the United Arab Emirates desert, where severe weather conditions may be experienced. In order to approach the subject, a computer model has been developed to enable the assessment of all phases of a laser warning receiver and missile seeker. MATLAB & SIMULINK software have been used to build the model. During this process experimentation and field trials have been carried out to verify the reliability of the model. This project enables both the evaluation and design of any generic laser warning receiver or missile seeker and specific systems if various parameters are known. Moreover, this model will be used as a guide to the development of reliable countermeasures for laser beamriding missiles. This paper (Part I of a four-part series of papers) outlines the theory required to construct a computer model for a laser beamriding missile engagement.