Simply put, if it has variable height (auto), it will never have overflow in the y axis (vertically), because the div will always grow to fit its contents.
overflow: scroll will force it to present a scrollbar, but it will always be disabled, because the contents will never extend beyond the displayed pane.
If you want vertical scrolling, you have to define a height, either in px, %, or em.
If you do height: 100%, the div will fill the height of the page, and scroll content that extends beyond the window's viewport height.
If you have a header area, try something like this:
body {
margin: 0;
}
#header {
position: absolute;
width: 100%;
height: 40%;
background-color: #ccc;
}
#body {
position: absolute;
top: 40%;
width: 100%;
height: 60%;
overflow-y: auto;
background-color: #eee;
}
<body>
<div id="header">
<p>Header</p>
</div>
<div id="body">
<p>Body</p><p>Body</p><p>Body</p><p>Body</p><p>Body</p><p>Body</p><p>Body</p>
<p>Body</p><p>Body</p><p>Body</p><p>Body</p><p>Body</p><p>Body</p><p>Body</p>
</div>
</body>
For a fixed-height header (per the comments), use absolute positioning with a top and botom value to position the scrollable div below it:
body {
margin: 0;
}
#header {
position: absolute;
width: 100%;
height: 60px;
background-color: #ccc;
}
#body {
position: absolute;
top: 60px;
bottom: 0px;
width: 100%;
overflow-y: auto;
background-color: #eee;
}
<body>
<div id="header">
<p>Header</p>
</div>
<div id="body">
<p>Body</p><p>Body</p><p>Body</p><p>Body</p><p>Body</p><p>Body</p><p>Body</p>
<p>Body</p><p>Body</p><p>Body</p><p>Body</p><p>Body</p><p>Body</p><p>Body</p>
</div>
</body>