Astrocytomas represent the most common form of glial tumors. The most malignant grade of these tumors, glioblastoma multiforme, may arise as a malignant progression from low-grade astrocytoma through anaplastic astrocytoma to secondary GBM, or else it may arise “de novo” as primary GBM. Both types of glioblastoma are usually histologically indistinguishable. However, distinct molecular alterations have been described between them that potentially allow differentiation between the two mechanisms of origin. Since malignant transformation is a multistep process, we summarize in this review the earliest genetic changes that seem to be involved in the appearance and development of low-grade astrocytic tumors, where early detection and treatment could be possible.