The article discusses the development and harmonization of a classification system for FTLD-TDP pathology. In 2006, two independent studies identified three distinct histological patterns in cases of frontotemporal lobar degeneration (FTLD) with ubiquitin-positive, tau-negative inclusions (FTLD-U). These studies, led by different groups, used different numbering systems for the subtypes. Subsequently, the identification of TDP-43 as the ubiquitinated pathological protein in most FTLD-U cases and sporadic amyotrophic lateral sclerosis (ALS) led to the recognition of a fourth subtype associated with inclusion body myopathy with Paget’s disease of bone and frontotemporal dementia (IBMPFD). To address the confusion caused by the two different classification systems, the authors propose a new, harmonized classification system for FTLD-TDP pathology. This new system uses letters to designate different pathological subtypes, based on their relative frequency and distinct morphological features. The goal is to improve communication and research in the field of FTLD diagnosis and pathology.The article discusses the development and harmonization of a classification system for FTLD-TDP pathology. In 2006, two independent studies identified three distinct histological patterns in cases of frontotemporal lobar degeneration (FTLD) with ubiquitin-positive, tau-negative inclusions (FTLD-U). These studies, led by different groups, used different numbering systems for the subtypes. Subsequently, the identification of TDP-43 as the ubiquitinated pathological protein in most FTLD-U cases and sporadic amyotrophic lateral sclerosis (ALS) led to the recognition of a fourth subtype associated with inclusion body myopathy with Paget’s disease of bone and frontotemporal dementia (IBMPFD). To address the confusion caused by the two different classification systems, the authors propose a new, harmonized classification system for FTLD-TDP pathology. This new system uses letters to designate different pathological subtypes, based on their relative frequency and distinct morphological features. The goal is to improve communication and research in the field of FTLD diagnosis and pathology.