  The settings for homotopical algebra---categories such as simplicial groups,
simplicial rings, $A_\infty$ spaces, $E_\infty$ ring spectra, etc.---are often
equivalent to categories of algebras over some monad or triple $T$. In such
cases, $T$ is acting on a nice simplicial model category in such a way that $T$
descends to a monad on the homotopy category and defines a category of homotopy
$T$-algebras. In this setting there is a forgetful functor from the homotopy
category of $T$-algebras to the category of homotopy $T$-algebras.
  Under suitable hypotheses we provide an obstruction theory, in the form of a
Bousfield-Kan spectral sequence, for lifting a homotopy $T$-algebra map to a
strict map of $T$-algebras. Once we have a map of $T$-algebras to serve as a
basepoint, the spectral sequence computes the homotopy groups of the space of
$T$-algebra maps and the edge homomorphism on $\pi_0$ is the aforementioned
forgetful functor. We discuss a variety of settings in which the required
hypotheses are satisfied, including monads arising from algebraic theories and
operads. We also give sufficient conditions for the $E_2$-term to be calculable
in terms of Quillen cohomology groups.
  We provide worked examples in $G$-spaces, $G$-spectra, rational $E_\infty$
algebras, and $A_\infty$ algebras. Explicit calculations, connected to rational
unstable homotopy theory, show that the forgetful functor from the homotopy
category of $E_\infty$ ring spectra to the category of $H_\infty$ ring spectra
is generally neither full nor faithful. We also apply a result of the second
named author and Nick Kuhn to compute the homotopy type of the space
$E_\infty(\Sigma^\infty_+ \mathrm{Coker}\, J, L_{K(2)} R)$.
