The late Leonard (Len) Muscatine (1932-2007) played a key role in the development of the understanding of algal-invertebrate symbioses. For over 40 years (1958-2005), Professor Muscatine was an inspirational mentor and leader in this field, guiding both the ideas and lives of generations of scientists, many of whom are still active in this research area. His scientific contributions were instrumental in crafting the understanding of a fundamentally important part of our world; that of endosymbiosis, where two or more independent organisms live together in a cellular harmony that belies a complex set of molecular and evolutionary interactions. Muscatine's research career was defined by investigations aimed at unraveling these interactions, particularly the specificity, metabolism, regulation, and disintegration of algal-invertebrate symbiosis. His gentle interrogation of his students and colleagues as to "What is the question?" led more than often to the focused research that yielded the insightful answers that still resonate today as the most current in the field.