newtheorem{theorem}{Theorem}
newtheorem{nono-theorem}{Theorem}[]

documentclass{article}
usepackage{amsthm}
theoremstyle{plain}

newtheorem*{theorem*}{Theorem}
newtheorem{theorem}{Theorem}

begin{document}
begin{theorem}  A numbered theorem.    end{theorem}
begin{theorem*} An unnumbered theorem. end{theorem*}
end{document}

documentclass{article}

usepackage{lipsum}
usepackage{ntheorem}
newtheorem{theorem}{Theorem}
newtheorem*{theorem-non}{Theorem}

begin{document}

begin{theorem}
lipsum[1]
end{theorem}
begin{theorem-non}
lipsum[1]
end{theorem-non}

end{document}

deftheorem{parnoindent{bf Theorem. } ignorespaces}
defendtheorem{}

defproblemstatement{parnoindent{bf Problem Statement. } ignorespaces}
defendproblemstatement{}

newcommand{bBox}{hbox{vrule width1.3ex height1.3ex}}
defproof{parnoindent{bf Proof. } ignorespaces}
defendproof{{ hspace*{fill}bBox parfillskip 0pt}smallskipnoindent}