Fog computing has gained significant attention for its potential to enhance resource management and service delivery by bringing computation closer to the network edge.While numerous surveys have explored various aspects of fog computing, there is a distinct gap in the literature when it comes to fog federation, a crucial extension that enables collaboration and resource sharing across multiple fog environments, enhancing scalability, service availability, and resource optimization.This paper provides a comprehensive survey of the existing work on fog federation, classifying the contributions from its inception to the present.We analyze the various approaches, architectures, and methodologies proposed for fog federation and identify the primary challenges addressed in this field.In addition, we explore the simulation tools and platforms utilized in evaluating fog federation systems.Our survey uniquely contributes to the literature by addressing the specific topic of fog federation, offering insights into the current state of the art and highlighting open research gaps and future directions.