Many bacteria have complex pleomorphic lifecycles — a feature particularly widespread across the class Alphaproteobacteria of the phylum Pseudomonadota. While research on bacteria with pleomorphic lifecycles has for many years focused on the dimorphic bacterium Caulobacter crescentus, more recent studies on less established alphaproteobacterial model bacteria have uncovered diverse variations of bacterial pleomorphism. Here, we provide an overview of the diversity and evolution of the complex lifecycles among dimorphic Alphaproteobacteria and highlight the presence of analogous lifecycles in unrelated bacteria across the bacterial domain. We discuss the commonalities and differences between dimorphic species, as well as the selective pressures that might have sculpted their lifecycles. Furthermore, we exemplify how the cellular appendages common among dimorphic Alphaproteobacteria, referred to as prosthecae, are not inherently linked to dimorphism. Finally, we highlight how the large diversity of dimorphic Alphaproteobacteria can be used to shed light onto the evolution of bacterial cell biology.