This book is concerned with the general theory of optimal estimation of parameters in systems subject to random effects and with the application of this theory. The focus is on choice of families of estimating functions, rather than the estimators derived therefrom, and on optimization within these families. Only assumptions about means and covariances are required for an initial discussion. Nevertheless, the theory that is developed mimics that of maximum likelihood, at least to the first order of asymptotics. The term quasi-likelihood has often had a narrow interpretation, associated with its application to generalized linear model type contexts, while that of optimal estimating functions has embraced a broader concept. There is, however, no essential distinction between the underlying ideas and the term quasi-likelihood has herein been adopted as the general label. This emphasizes its role in extension of likelihood based theory. The idea throughout involves finding quasi-scores from families of estimating functions. Then, the quasilikelihood estimator is derived from the quasi-score by equating to zero and solving, just as the maximum likelihood estimator is derived from the likelihood score. This book had its origins in a set of lectures given in September 1991 at the 7th Summer School on Probability and Mathematical Statistics held in Varna, Bulgaria, the notes of which were published as Heyde (1993). Subsets of the material were also covered in advanced graduate courses at Columbia University in the Fall Semesters of 1992 and 1996. The work originally had a quite strong emphasis on inference for stochastic processes but the focus gradually broadened over time. Discussions with V.P. Godambe and with R. Morton have been particularly influential in helping to form my views. The subject of estimating functions has evolved quite rapidly over the period during which the book was written and important developments have been emerging so fast as to preclude any attempt at exhaustive coverage. Among the topics omitted is that of quasi-likelihood in survey sampling, which has generated quite an extensive literature (see the edited volume Godambe (1991), Part 4 and references therein) and also the emergent linkage with Bayesian statistics (e.g., Godambe (1994)). It became quite evident at the Conference on Estimating Functions held at the University of Georgia in March 1996 that a book in the area was much needed as many known ideas were being rediscovered. This realization provided the impetus to round off the project rather vi PREFACE earlier than would otherwise have been the case. The emphasis in the monograph is on concepts rather than on mathematical theory. Indeed, formalities have been suppressed to avoid obscuring "typical" results with the phalanx of regularity conditions and qualifiers necessary to avoid the usual uninformative types of counterexamples which detract from most statistical paradigms. In discussing theory which holds to the first order of asymptotics the treatment is especially informal, as be...