Free light chains (FLC) of immunoglobulins have been of interest to researchers in various branches of medicine since their discovery in the late 19th and early 20th centuries. In addition to hematology, where the role of monoclonal FLC (mFLC) produced by the clone of the B-cell line is being actively studied, other specialties are no exception. Thus, in modern neurology and rheumatology, polyclonal FLC (pFLC), produced by B-lymphocytes during their excessive immune/autoimmune stimulation, are being actively studied. In the pathogenesis of kidney disease, both mFLC and pFLC can be involved. The importance of mFLC for nephrology is associated, firstly, with various variants of kidney damage in monoclonal gammopathies - cylinder nephropathy, AL-amyloidosis, etc., and secondly, with the initiation of the epithelial-mesenchymal transition and the progression of sclerotic changes in the renal tubulointerstitium. With regard to pFLC, their increased level in kidney pathology of various origins is associated with an unfavorable prognosis not only in relation to the progression of chronic kidney disease but also in life. This allows us to reasonably assume the participation of PSLC in the initiation of profibrotic processes in the kidney. Currently, it is believed that the mechanism of epithelial-mesenchymal transition, which underlies the formation of fibrosis of the renal parenchyma, can be mediated not only by mFLC, but also by pFLC, which has been demonstrated in a limited number of studies in some glomerulopathies. The review outlines the current understanding of FLC, as well as the role of mFLC and pFLC in renal pathology.