Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that has no effective treatment. To date, ALS is considered as a multifactorial heterogeneous disease, in which the death of motor neurons is a final result of the different pathological pathways. Modern diagnostic criteria and classification of ALS do not take into account all heterogeneity of the disease. Despite the development of molecular neurobiology and neurophysiology, genetics, and technology, significant progress in understanding the pathogenesis of ALS, the disease is diagnosed primarily on the basis of clinical manifestations. In recent years, a number of clinical trials of promising drugs have failed to show positive results. Among the reasons for these failures are variability of ALS forms, patients enrollment already at a late stage of the disease, the lack of use of biomarkers for patients selection and drugs’ pharmacodynamics assessment. The study of biomarkers and their implementation in clinical practice can help to solve these problems. Here we will discuss the fluid-based biomarkers for ALS.