Heterogeneous ice nucleation at solid surfaces is important in many physical systems including the Earth's atmosphere. However, despite the importance of ice nucleation, the particular features of a surface that are necessary to promote ice nucleation, and the molecular mechanisms involved remain poorly understood. We have recently used molecular dynamics simulations to investigate two model surfaces that are very effective ice nucleating agents. The first is a general model that illustrates the strong ice nucleation influence of local electric fields, and the second is an atomistically detailed surface designed to represent AgI, which is among the most effective ice nucleating substances known. The talk will emphasize the microscopic mechanisms of ice nucleation in these particular situations.