The general aim of this dissertation is to gain insight into the physiotherapeutic validity of physiotherapy research in subjects with non-specific neck pain. Chapter 1 describes the background of the research and the research questions and gives an overview of the studies performed. Chapter 2 presents the results of a systematic review (SR) of the completeness of the clinical reasoning process within the methodology of the RCT in patients with non-specific neck pain. For the SR analysis 122 studies were included. In the majority of studies (70%) the described clinical reasoning process was incomplete. There was scarcely any association between the degree of risk of bias and the completeness of the clinical reasoning process, indicating that better methodological quality does not necessarily imply a better description of clinical reasoning process. Chapter 3 presents the results of a SR in which we sought to identify published classification systems with a targeted treatment approach (treatment-based classification systems (TBCSs)) for patients with non-specific neck pain. Thirteen TBCSs were identified. In conclusion, existing treatment-based classification systems are of moderate quality at best. Moreover, these systems were not more effective than alternative treatments. Therefore, we do not recommend the use of these systems in daily physiotherapy practice. Chapter 4 describes a Delphi study of the clinical reasoning process of physiotherapy experts in unimodal interventions in patients with non-specific neck pain. This study had three goals. First, we aimed explore the expert opinions on the indication for physiotherapy when, other than neck pain, there are no positive signs and symptoms, no positive diagnostic tests or complaints of limitations in functioning or restrictions in participation. Second, we focused on the experts' use of measurement tools and when they are used to support and objectify the clinical reasoning process. Finally, we wanted to reach consensus among experts on the use of unimodal interventions in patients with non-specific neck pain. According to all experts, pain alone was not considered to be an indication for physiotherapy. Patient reported outcome measures were mainly used for evaluative purposes and physical tests for diagnostic and evaluative purposes. Only 6 of the 18 variants of sequential linear clinical reasoning reached a consensus of more than 50%. Chapter 5 describes a review that examined the completeness of the description of manipulation and mobilization interventions in randomized controlled trials of subjects with non-specific neck pain. In conclusion, mobilization or manipulation interventions are poorly reported in RCTs, compromising the external validity of RCTs, making it difficult for clinicians and researchers to replicate these interventions. Chapter 6 investigated the diagnostic physiotherapeutic process regarding limited ROM of the neck. It can be concluded that the overall diagnostic accuracy of physical examination is limited (compared to the CROM measurement). Therefore, a measurement device should be used in daily physical therapy practice to assess if a movement direction is restricted. Chapter 7 describes an exploratory, practice-oriented study into matched treatments in patients with non-specific neck pain. The objective of this study was 1) to establish the measurement error of the used accelerometer; 2) To determine which different treatments are used; 3) To explore if the cervical ROM, pain, (perceived) disability and motor control improved after one treatment. The SCT is a reliable accelerometer for measuring neck ROM, with a small measurement error. Eight different treatments were carried out. Pain, disability and left and right rotation showed a clinically relevant improvements (exceeded the measurement error). Chapter 8 comprises the general discussion. The general discussion presents an overview of this dissertation and discusses the strengths and limitations of the studies and possible implications of the results and recommendations for future research.