With the move towards a converged, all-IP next generation network (NGN) offering end users personalized, advanced multimedia services anywhere and anytime, a key challenge to be addressed is support for the service-level negotiation and dynamic adaptation of Quality of Service (QoS) parameters. Considering that the majority of existing approaches presented in literature and standards focus on solving particular aspects of this problem, it may be concluded that there is a need for a comprehensive and generalized approach addressing and relating various business and technical aspects. A generic framework has been proposed referred to as the SErvice-Level E2E QoS negoTiation and adaptation (SELEQT) framework. The framework is independent of a particular service and network scenario and is comprised of four different conceptual models addressing and relating the following aspects: the actors involved in QoS negotiation (Business Model); specification of the parameters that impact the QoS negotiation process, i.e., the parameters that serve as input and the parameters that comprise the output (Data Specification Model); the identification of dynamic adaptation-triggering events that lead to QoS renegotiation and adaptation (Adaptation Event Model); and the complex process of matching and coordinating QoS parameters specified and signaled by involved actors (Negotiation Model). In order to address the issue of applying the SELEQT framework in practice, a methodology is provided in the form of guidelines comprised of key actions to be taken, considered from the perspective of various involved actors. Guidelines include the modeling of a novel functionality named the QoS Matching and Optimization Function (Q-MOF), introduced for the purpose of providing QoS parameter matching and optimization logic dynamically during the course of a service lifetime. The proposed framework and methodology are demonstrated in a case study involving E2E service level QoS negotiation and adaptation in a network architecture based on the IP Multimedia Subsystem (IMS) as specified by the 3GPP. Key functionalities needed to illustrate framework applicability have been implemented in the scope of a prototype in a laboratory testbed, and prototype performance measurements have been conducted.