The prediction of the population level consequences of impacts on marine mammals as a result of proposed marine developments is a crucial part of the impact assessment and decision making process. A variety of different population modelling approaches have been used in recent years to provide information for consenting decisions about the potential magnitude and significance of impacts. The range of models and approaches that have been adopted and presented in Environmental Statements and Habitat Regulations Appraisal (HRA) reports are quite complex and the variety can be confusing to the non-specialist. This report is intended to be an accessible summary reference guide to marine mammal population modelling for statutory nature conservation body (SNCB) advisers and practitioners dealing with assessments of the potential impacts on marine mammal populations. It is also intended for SNCB marine mammal specialists to use as a resource to inform approaches to decision making and planning and carrying out strategic level assessments. This guide first provides an overview of the main generic types of approaches used in population assessment/decision making (Section 2) before exploring a few specific examples in more detail in Section 3. Section 4 provides an overview of the main issues arising from this overview and examples and comments on recent developments and future directions. A glossary of terms is provided at the end of the guide.