Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the loss of upper and lower motor neurons. Despite the availability of three FDA-approved drugs to slow functional decline, there is no cure for ALS. The need for biomarkers to improve patient care and accelerate treatment development is critical. This article reviews the efforts in identifying various fluid biomarkers, including diagnostic, prognostic, susceptibility/risk, and response biomarkers, to enhance ALS diagnosis, prognosis, and clinical trial design. Over 20 years, several promising fluid biomarker candidates have emerged, with neurofilaments and TDP-43 being among the most studied. Neurofilaments, particularly neurofilament light (NFL) and phosphorylated neurofilament heavy (pNfH), are widely used as markers of neuronal injury and degeneration, showing potential in diagnosing, predicting prognosis, and monitoring treatment response. TDP-43, a proteinopathy central to ALS pathology, has also been explored as a biomarker, with studies examining its levels and structural changes in CSF and plasma. Despite challenges, these biomarkers offer significant potential to improve ALS management and therapeutic development. Future research aims to refine these biomarkers and explore new strategies for more accurate and timely diagnosis and treatment.Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the loss of upper and lower motor neurons. Despite the availability of three FDA-approved drugs to slow functional decline, there is no cure for ALS. The need for biomarkers to improve patient care and accelerate treatment development is critical. This article reviews the efforts in identifying various fluid biomarkers, including diagnostic, prognostic, susceptibility/risk, and response biomarkers, to enhance ALS diagnosis, prognosis, and clinical trial design. Over 20 years, several promising fluid biomarker candidates have emerged, with neurofilaments and TDP-43 being among the most studied. Neurofilaments, particularly neurofilament light (NFL) and phosphorylated neurofilament heavy (pNfH), are widely used as markers of neuronal injury and degeneration, showing potential in diagnosing, predicting prognosis, and monitoring treatment response. TDP-43, a proteinopathy central to ALS pathology, has also been explored as a biomarker, with studies examining its levels and structural changes in CSF and plasma. Despite challenges, these biomarkers offer significant potential to improve ALS management and therapeutic development. Future research aims to refine these biomarkers and explore new strategies for more accurate and timely diagnosis and treatment.