The development of our understanding of frontotemporal dementia (FTD) has gathered pace over the last 10 years. After taking a back seat to Alzheimer's disease for many years FTD has emerged as a significant group of heterogeneous diseases often affecting people under the age of 65. FTD has also been brought into the spotlight as the major disease entities of the group have clinical, genetic and pathological links to motor neuron disease/amyotrophic lateral sclerosis, indicating that they form a disease spectrum. In this review, we overview how the pathological concept of frontotemporal lobar degeneration (FTLD) and the clinical concept of FTD evolved and show that FTLD, once thought of as a single disorder, represents a heterogeneous group of diseases with overlapping clinical symptoms, multiple causative genes and varying underlying pathology. We also provide a brief summary of the clinical manifestations, summarize the major genetic aspects and describe the main pathological features seen in the different subtypes of FTLD. We also summarize the correlations that exist between clinical presentations and pathological variants. An overview of the main pathogenic mechanisms is also provided.