Doctrine provides a military organization with unity of effort and a common philosophy, language, and purpose. This field manual provides doctrine for the application of engineer reconnaissance capabilities in support of the combined arms team conducting full spectrum operations. Engineer reconnaissance, like chemical, biological, radiological, and nuclear (CBRN) and other technical applications, is not a form of reconnaissance (see chapter 3 for a discussion of the four forms of reconnaissance). Engineer reconnaissance is instead a focused application of special/unique capabilities supporting reconnaissance operations and is applicable over/pertinent to all four forms of reconnaissance. Field manual (FM) 3-34.170/Marine Corps Warfighting Publication (MCWP) 3-17.4 updates the FM that provides doctrinal guidance for engineer reconnaissance in support of full spectrum operations, including engineer reconnaissance in support of tactical operations as well as engineer technical reconnaissance support. This manual supersedes FM 5-170 and supports the doctrine found in FM 3-0, FM 3-34, and FM 6-0, and Field Manual Interim (FMI) 5-0.1. This manual will serve as a reference document for engineer commanders and staff, leaders, training developers, and doctrine developers throughout the Army and Marine Corps. It will also provide guidance to commanders for the employment of engineer reconnaissance capabilities in support of all operations. It is also the primary reference for engineer reconnaissance for Joint Publication (JP) 3-34. This FM has an introduction and six chapters. It includes significant discussion on integrating the planning for engineer reconnaissance support within the planning doctrine in FM 5-0 and the command and control (C2) doctrine in FM 6-0. The introduction expands upon the manual's purpose and summarizes the doctrinal changes it contains. Chapter 1 provides a doctrinal framework for the provision of engineer reconnaissance capabilities resident within engineer functions and supporting the warfighting functions, describes a range of tactical to technical engineer reconnaissance capabilities, and provides capabilities and limitations of the engineer reconnaissance team (ERT). Chapter 2 provides doctrine for integrating the planning for engineer reconnaissance within information management and planning processes of the combined arms team. It specifically addresses integration of geospatial support and provides specific C2 considerations for integration of engineer reconnaissance. Chapter 3 provides doctrine for integrating the application of engineer reconnaissance within tactical reconnaissance operations of the combined arms team. It also addresses considerations for the sustainment of engineer reconnaissance elements. Chapter 4 provides doctrine for the conduct of ERT operations providing engineer reconnaissance support at the tactical end of the range described in Chapter 1. ERTs conduct zone, area, and route reconnaissance with a specified additional focus on required technical information. Chapters 5 and 6 provide doctrine for the conduct of engineer assessments and surveys which provide engineer reconnaissance support at the technical end of the range described in Chapter 1. Assessment and survey teams conduct reconnaissance specifically focused on collecting detailed technical information. Appendix B illustrates the preparation of required engineer reconnaissance reports and forms. Appendix C incorporates the smartcard tools developed for infrastructure assessment. Appendix D provides the environmental baseline assessment tool. Appendix H includes a collection of other useful tools and resources.