The equations of motion of classical mechanical systems with m degrees of freedom have usually the form \(A_\sigma (t,\,q^\mu ,\,\dot{q}^\mu ) + B_{\sigma \nu }(t,\,q^\mu ,\,\dot{q}^\mu )\ddot{q}^\nu = 0\) , \(1\le \sigma ,\,\nu ,\,\mu \le m\) , i.e. they form the system of second order ordinary differential equations. We can consider the left-hand sides of these equations as components \(E_\sigma (t,\,q^\mu ,\,\dot{q}^\mu ,\,\ddot{q}^\mu )\) of the dynamical form \(E=E_\sigma \,\textrm{d}q^\sigma \wedge \,\textrm{d}t\) . Nevertheless, the form E need not necessarily come from a Lagrangian. So, the problem is to find the criteria under what the dynamical form E affine in accelerations is variational, i.e. it comes from a Lagrangian. If the answer is positive, there is a second task—to find all Lagrangians corresponding to this dynamical form. The Vainberg-Tonti Lagrangian is constructed. The Lagrangian of the minimum possible order is searched, called the minimal Lagrangian. The just formulated problem is called the inverse problem of calculus of variations. In this chapter we present the complete solution of the local version of this problem for second order dynamical forms affine in accelerations. The variationality of first order dynamical forms will be discussed as well. Examples are presented.