Markdown provides backslash escapes for the following characters:

\   backslash
`   backtick
*   asterisk
_   underscore
{}  curly braces
[]  square brackets
()  parentheses
#   hash mark
+   plus sign
-   minus sign (hyphen)
.   dot
!   exclamation mark

For example, # Big Greeting will be rendered as a big header by the Markdown rule.
However, when you want to literally display the leading hash mark, prepend a backslash like so:

 \# Big Greeting