The emergence of the telencephalon as a forebrain part with a complex structure is one of the mostimportant aromorphoses in vertebrate evolution. The telencephalon developed and improved in evolution toallow higher nervous activity forms observed in animals and humans. A telencephalic anlage is separated atthe earliest stages of vertebrate ontogenesis, when the anterior part of the neural tube differentiates into threecerebral vesicles: the prozencephalon as an anlage of the future forebrain, the mesencephalon as the futuremidbrain, and the rhombencephalon as the future hindbrain. The forebrain further differentiates to form thetelencephalon and the diencephalon. The development of brain structures and regions is modulated by theexpression of certain regulatory genes, which code for transcription factors and signaling molecules. Problemsof the evolutionary origin and ontogenesis of the telencephalon are still poorly understood at the molecularlevel, although they are among central problems of modern developmental biology. Recent studies of theevolutionary mechanisms responsible for the emergence of the telencephalon in vertebrates have paid muchattention to cyclostomes (lampreys and hagfishes) as the most evolutionarily ancient vertebrate groups andTunicata (ascidians) and Cephalochordata (lancelets) as the closest relatives of vertebrates. Cyclostomes areof particular interest because they were the first in evolution to have the telencephalon as a separate morphologicalstructure and because they might preserve the expression patterns and regulatory mechanisms characteristicof vertebrate ancestors. The review summarizes and analyzes the data accumulated in recent yearsfrom studies of the genetic mechanisms of early telencephalon development in lower vertebrates and searchesfor telencephalon homologs in two vertebrate-related chordate groups, Cephalochordata and Tunicata.