Prior research indicates that peer code review is an effective method for reducing the number of defects and improving the quality of code. Besides maintaining the integrity of the code, code review spreads knowledge, expertise, and development techniques among the review participants. In the recent years, many Open Source Software (OSS) communities and commercial organizations have adopted 'contemporary' or 'modern' code review, an informal, regular, tool-based process. Because both OSS and commercial developers spend a significant amount of effort performing code reviews, the primary goal of this dissertation is to better understand contemporary code review, its non-technical benefits, factors that influence its outcomes, and its impact on peer impression formation. To address this goal, this dissertation describes empirical studies using surveys, software repository mining, and social network analysis. The first study is a survey of OSS developers to understand their collaboration and the process by which they form impressions of each other. The results suggest that coding-related factors influence impression formation the most among OSS developers. Therefore, the types of interactions where participants can judge a peer's code or creativity (e.g., code review) should be crucial for peer impression formation. The results of this study motivated the selection of peer code review as the focus of this dissertation. The second study describes a survey of developers from 36 popular OSS projects and from Microsoft about: 1) the code review process in their projects, 2) their expectations from code review, and 3) how code review impacts impressions about their peers. The results suggest that the primary perceived benefit of code review is knowledge sharing, relationship building, better designs, and ensuring maintainable code, as opposed to the expected result of defect detection. Code reviews help build impressions between code review participants. Those impressions not only impact the code review process but also future collaborations among developers. Due to the rarity of face-to-face interactions, OSS developers rely more on the reputation of and relationship with the author during code reviews. Conversely, Microsoft developers focus more on expertise and anticipated efforts. Finally, the third study aims to find the impact of developers' reputation on the outcome of his/her code review requests. The results suggest that developers' reputations help them receive quicker feedback on their review requests, complete reviews in shorter time, and get their code changes accepted. Newcomers to OSS projects suffer the most due to delayed feedback, which may discourage their future participation. A reviewer recommendation system to triage incoming code review requests can be useful to reduce delayed feedback for newcomers. Based on the results from these studies, this dissertation makes recommendations for practitioners to adopt and improve code review practices.