1 Introduction.- 2 Material and Methods.- 3 Observations from the Author’s Investigations.- 3.1 General Comments.- 3.2 Results of Large Nonsuperficial Horseradish Peroxidase Injections.- 3.3 Results of Restricted Cortical Horseradish Peroxidase Injections.- 4 Review of the Literature and Discussion of Brain Stem Afferents to the Cerebellar Cortex from Particular Precerebellar Nuclei.- 4.1 Pontine Nuclei, Nucleus Reticularis Tegmenti Pontis, and Corpus Pontobulbare.- 4.2 Raphe Nuclei.- 4.3 A Lateral Tegmental Cell Group at the Level of the Isthmus.- 4.4 Locus Coeruleus.- 4.5 Trigeminal Nuclei.- 4.6 Vestibular Nuclei.- 4.7 Perihypoglossal Nuclei.- 4.8 Inferior Olive.- 4.9 Lateral Reticular Nucleus.- 4.10 Paramedian Reticular Nucleus and Nucleus Interfasciculares Hypoglossi.- 4.11 External Cuneate Nucleus.- 4.12 Main Cuneate Nucleus.- 4.13 Gracile Nucleus.- 4.14 Nucleus of the Solitary Tract.- 4.15 Other Brain Stem Nuclei Projecting to the Cerebellar Cortex.- 5 General Discussion of the Afferent Organization of the Cerebellar Cortex of the Cat.- 5.1 Overlap and Segregation of Brain Stem Areas Projecting to Different Cerebellar Lobules.- 5.2 Overlap and Segregation of Afferents from Different Brain Stem Areas Within the Cerebellar Cortex.- 5.3 Sagittal Organization of the Cerebellar Cortex.- 5.4 Afferent Organization of the Cerebellar Cortex in Relation to Concepts of Functional Localization.- 6 Summary.- References.