In this talk we discuss the finite element approximation of fluid-structure interaction problems. After presenting a finite element version of the Immersed Boundary Method (originally developed by Peskin in the finite difference framework), we show how the introduction of a distributed Lagrange multiplier allows the design of a Fictitious Domain formulation. The properties of our approach include a superior mass conservation, unconditional stability for the time discretization, and rigorous convergence analysis for the steady state solution.