 Jonathan Slack’s Essential Developmental Biology (Slack 2006, 2013) is organized around four main types of processes, also described as clustered groups of problems, which occur during embryonic development: regional specification (pattern formation), cell differentiation, morphogenesis, and growth. These broad clusters are then fleshed out along a standard timeline of early development, highlighting gametogenesis, fertilization, cleavage, gastrulation, and axis specification  (see Section 1.2).  Different experimental approaches (cell and molecular biology, developmental genetics, and experimental embryology) are utilized in a specific set of model organisms  (see below, Section 4)  to dissect the workings of these developmental phenomena. Subsequent chapters cover later aspects of development (e.g., organogenesis), with different systems treated in depth by tissue layer, differentiation and growth, or in relation to evolutionary questions (see below,  Section 5).  Throughout this presentation, no specific theory, set of hypotheses, or dominant model is invoked to organize these different domains of investigation. Instead, broad clusters of questions that reflect generally delineated processes (differentiation, specification, morphogenesis, and growth) set the agenda of research.