This chapter examines the critical role of post-tender cost management and negotiation tactics in ensuring construction project success. In an industry characterized by complexity and financial risks, effective cost control after contract award is essential for maintaining project viability and profitability. The chapter provides a comprehensive framework for understanding post-tender cost management, including its definition, scope, and key stakeholders. It explores strategies for effective cost management, including establishing robust cost management plans, budgeting and forecasting techniques, resource optimization, and implementing cost control measures. The text details methods for tracking, monitoring, and analyzing cost variances, emphasizing the importance of technology integration. Negotiation tactics with clients, subcontractors, and suppliers are examined as essential skills for managing financial expectations and resolving disputes. The chapter concludes with best practices and future trends, highlighting the evolving role of technology, sustainability considerations, and collaborative contracting models in shaping cost management. Throughout, the focus remains on practical applications that construction professionals can implement to enhance financial control and project outcomes.