The pervasive systems are weaving themselves in our daily life, making it possible to collect user information invisibly, in an unobtrusive manner by even unknown parties. So run time trustworthiness calculation would be a major issue in these environments. The huge number of interactions between users and pervasive devices necessitate a comprehensive trust model which unifies different trust factors like context, recommendation, and history to precisely calculate trustworthiness of each party. In this paper we review previous trust models in pervasive systems and analyze them to highlight the importance of our research, and then we introduce a Unified Trust Model (UTM) which calculates entities trustworthiness based on history, recommendation, and context and formally use these factors in trustworthiness calculation. Finally we describe our future research plan to add platform properties using Trusted Platform Modules (TPM) in calculating trustworthiness.