This comprehensive narrative review provides an overview of the current scientific evidence regarding maxillary sinus hypoplasia (MSH). This review highlights several variants of MSH that are relevant to implant planning and treatment in daily clinical practice. MSH is characterized by a reduction in sinus volume, which contrasts with maxillary sinus pneumatization, a condition that has received more clinical attention. Nevertheless, certain types of MSH can significantly impact implant surgery and the management of associated complications. The maxillary sinus volume can be affected by factors such as infection, trauma, genetic predispositions, and changes within the sinus or adjacent anatomical structures, including the nasal cavity and the ethmoid sinus. In cases of MSH, the maxillary sinus floor is positioned more cranially than the nasal floor, and the distance between the lamina papyracea of the eye and the middle meatus antrostomy point increases horizontally. Several variants of MSH potentially affect implant therapy. Chronic maxillary sinus atelectasis, which results from persistent ostium blockage, may have clinical implications, particularly when implant-related infections occur in the MS. Inferior meatus pneumatization involves a lateral expansion of the nasal cavity, requiring a nasal floor elevation procedure when placing implants in the posterior maxilla. Additionally, the formation of an ethmomaxillary sinus may create a septum-like structure in the distal area of the maxillary sinus, necessitating two separate bone access windows during the sinus floor elevation procedure. Given these considerations, it is crucial for clinicians to be aware of MSH. Accurate diagnosis of MSH requires the use of 3-dimensional radiography techniques, such as cone-beam computed tomography.