Parkinsonian disorders (PS) are a heterogeneous group of neurodegenerative diseases characterised by the presence of parkinsonism. Parkinson’s disease (PD) is the most common PS, characterised by the accumulation of alpha-synuclein (αSyn) in Lewy bodies, and the presence of motor features – tremor, bradykinesia, and rigidity. Similarly, dementia with Lewy bodies (DLB) is among the most prevalent PS and dementias, having both αSyn inclusions and frequent Alzheimer’s disease (AD) co-pathology, therefore often exhibiting both parkinsonism and cognitive impairment. Together with PD, these diseases are commonly known as Lewy body diseases (LBD). Other PS have distinct underlying pathologies. While PD, DLB, and multiple system atrophy (MSA) have αSyn as their primary proteinopathy, progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) have tau as a main hallmark. Despite these differences, these disorders exhibit similar clinical features, which presents a significant challenge in achieving an accurate clinical diagnosis, and often results in high misdiagnosis rates. Fluid biomarkers have emerged as a robust diagnostic tool to improve diagnosis, which is desperately needed in the PS field. The main aim of this thesis was to address the need for novel biomarkers in the PS field, focusing on LBD, by investigating multiple biomarker candidates – αSyn, corticotropin-releasing hormone (CRH), and the neuronal pentraxin 2 (NPTX2)/phospho-tau (p-tau) ratio. The levels of potential biomarkers were analysed in cerebrospinal fluid (CSF) across different neurodegenerative diseases, using different methodologies, including the αSyn seed amplification assay (αSyn-SAA), proximity extension assay (PEA, Olink platform), targeted mass spectrometry (parallel reaction monitoring, PRM), and single molecule array (Simoa) assays. One of the main findings related to the αSyn-SAA, which was confirmed as a robust tool to detect αSyn seeds in CSF and able to accurately identify and discriminate PD and MSA patients from PS with no αSyn involvement, such as PSP and CBD. It was also able to detect possible αSyn co-pathology in patients whose primary pathology was not αSyn, as well as identify clinically diagnosed PD patients who may have been misdiagnosed. Another finding concerned CRH and its precursor (pro-CRH), which are known to be involved in stress response. CSF CRH was identified in a large proteomics PEA study and found to be selectively decreased in LBD and atypical PS (MSA and PSP), discriminating these from other types of dementia. Similarly, pro-CRH, for which a novel PRM assay was developed, was decreased in LBD patients, recapitulating the CRH findings. The decrease in CRH was additionally linked to cognitive impairment as well as inflammation and neuropsychiatric measures. This study confirmed CRH as a promising biomarker and prompted further investigations into the underlying mechanisms of CRH in PS and its relevance as a biomarker in these diseases. Finally, the clinical utility of the NPTX2/p-tau ratio compared to the individual markers was assessed in AD, DLB, and frontotemporal lobar degeneration related syndromes (FTLDrs), a group of disorders characterised by degeneration of the frontal and temporal lobes, using a newly developed Simoa assay. Overall, NPTX2/p-tau outperformed both biomarkers separately, being able to not only discriminate AD, DLB, and FTLDrs from healthy individuals, but also from each other. Its relationship with cognition was also stronger than NPTX2 or p-tau, being a particularly good biomarker to detect specific cognitive deficits relevant in DLB. In conclusion, this thesis emphasised the need for novel biomarkers in PS by addressing the complexity of these disorders, where similar clinical features and mixed co-pathologies can significantly hinder the diagnostic process. It also underscored the potential of several biomarkers across the PS spectrum and their clinical usefulness, underscoring that their worth goes beyond diagnostic accuracy, as they may serve other purposes in monitoring disease progression, predicting cognitive decline, as well as improving disease characterisation.