This book discusses the ways in which the algebras in a locally finite quasivariety determine its lattice of subquasivarieties.  The book starts with a clear and comprehensive presentation of the basic structure theory of quasivariety lattices, and then develops new methods and algorithms for their analysis.  Particular attention is paid to the role of quasicritical algebras.  The methods are illustrated by applying them to quasivarieties of abelian groups, modular lattices, unary algebras and pure relational structures.  An appendix gives an overview of the theory of quasivarieties.  Extensive references to the literature are provided throughout.