  Let $R\subset F$ be an extension of real closed fields and ${\mathcal
S}(M,R)$ the ring of (continuous) semialgebraic functions on a semialgebraic
set $M\subset R^n$. We prove that every $R$-homomorphism $\varphi:{\mathcal
S}(M,R)\to F$ is essentially the evaluation homomorphism at a certain point
$p\in F^n$ \em adjacent \em to the extended semialgebraic set $M_F$. This type
of result is commonly known in Real Algebra as Substitution Theorem. In case
$M$ is locally closed, the results are neat while the non locally closed case
requires a more subtle approach and some constructions (weak continuous
extension theorem, \em appropriate immersion \em of semialgebraic sets) that
have interest on their own. We afford the same problem for the ring of bounded
(continuous) semialgebraic functions getting results of a different nature.
