This book aims to clarify some puzzles that have been topical in formal linguistics for many decades now. It concerns the status of English and, for comparison, Czech infinitives, as opposed to finite clauses and marginally also gerunds. The book systematically demonstrates the functional (meaning) and syntactic (form) properties of infinitives. It highlights the essential properties common to all infinitives, their core structure in Logical Form (LF) and their relation to an irrealis feature. It also investigates the particularities of the various infinitival constructions.