Large-scale complex Information Technology (IT) Systems Implementations, such as Enterprise Resource Planning (ERP) implementations, with significant change management, often yield minimal return on investment (ROI); such projects often fail. This study aims to focus on the role, impact, and value of external professional services consultants within the ecosystem of large complex IT systems and ERP implementations. Each chapter will provide an extensive examination of various contributing factors and possible corrective approaches, starting with the theoretical arguments underpinning resource-based theory and its possible relationship to IT project success, with the involvement of external Professional Services Consultants. The study will rely on a qualitative approach to examine the attributes of successful and failed IT projects, by comparing internal resources, external professional services consultant’s, and partnership models of project implementations.