  In this paper we consider Bernstein's Lethargy Theorem (BLT) in the context
of Fr\'{e}chet spaces. Let $X$ be an infinite-dimensional Fr\'echet space and
let $\mathcal{V}=\{V_n\}$ be a nested sequence of subspaces of $ X$ such that $
\bar{V_n} \subseteq V_{n+1}$ for any $ n \in \mathbb{N}$ and $
X=\bar{\bigcup_{n=1}^{\infty}V_n}.$ Let $ e_n$ be a decreasing sequence of
positive numbers tending to 0. Under an additional natural condition on
$\sup\{\{dist}(x, V_n)\}$, we prove that there exists $ x \in X$ and $ n_o \in
\mathbb{N}$ such that $$ \frac{e_n}{3} \leq \{dist}(x,V_n) \leq 3 e_n $$ for
any $ n \geq n_o$. By using the above theorem, we prove both Shapiro's
\cite{Sha} and Tyuremskikh's \cite{Tyu} theorems for Fr\'{e}chet spaces.
Considering rapidly decreasing sequences, other versions of the BLT theorem in
Fr\'{e}chet spaces will be discussed. We also give a theorem improving
Konyagin's \cite{Kon} result for Banach spaces.
