Within this work, we consider optimization settings for nonlinear, nonstationary fluid-structure interaction. The problem is formulated in a monolithic fashion using the arbitrary Lagrangian-Eulerian framework to set-up the fluid-structure forward problem. In the optimization approach, either optimal control or parameter estimation problems are treated. In the latter, the stiffness of the solid is estimated from given reference values. In the numerical solution, the optimization problem is solved with a gradient-based solution algorithm. The nonlinear subproblems of the FSI forward problem are solved with a Newton method including line search. Specifically, we will formally provide the backward-in-time running adjoint state used for gradient computations.Our algorithmic developments are demonstrated with some numerical examples as for instance extensions of the well-known fluid-structure benchmark settings and a flapping membrane test in a channel flow with elastic walls. arXiv:1910.03424v1 [math.NA] 8 Oct 2019 [53,57,60,12]. Here, we notice that the required adjoints are the same as used for adjoint-based error estimation; see for instance [62,51]. Nonlinear (stationary) FSI investigating various partitioned coupling techniques was recently subject in [54]. The by far more challenging situation of nonstationary settings is listed in the following. A nonstationary situation assuming a rigid solid was theoretically studied in [48]. Further theoretical results for a boundary control FSI problem were established in [8].Parameter estimation to detect the stiffness of an arterial wall with a well-posedness analysis and numerical simulations was addressed in [49]. Again in blood flow simulations, a data assimilation problem was formulated in [33], in which however, the arterial walls were not considered. A full FSI problem for data assimilation using a Kalman filter was subject in [6]. In [44], the authors used optimization techniques to formulate the FSI coupling conditions. Adjoints for 1D FSI were derived in [16,47]. Reduced basis methods for FSI-based optimization were developed in [45]. Optimal control of nonstationary FSI applied to benchmark settings was investigated in [3]. A linearized FSI optimization problem was addressed in [23] and detailed results for full-time-dependent FSI optimal control were summarized in [22]. In this respect, we also mention [24] in which the adjoints required for optimization were employed for dual-weighted residual error estimation for time adaptivity. Most recently, a uncertainty quantification framework for fluid-structure interaction with applications in aortic biomechanics was developed in [43].The significance of the current work is on the development of a robust fully monolithic formulation for gradient-based optimization for nonstationary, nonlinear FSI problems. Here, the coupled problem is prescribed in the reference configuration with the help of the ALE approach in a variationalmonolithic way. As previously summarized in our literature review, only very few results exis...