diff --git a/DESCRIPTION b/DESCRIPTION
index 76d439115..4aa2128b1 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -60,6 +60,7 @@ Collate:
     'card.R'
     'deprecated.R'
     'files.R'
+    'fill.R'
     'imports.R'
     'layout.R'
     'nav-items.R'
diff --git a/NAMESPACE b/NAMESPACE
index 47eb47277..a52f39361 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -1,5 +1,9 @@
 # Generated by roxygen2: do not edit by hand
 
+S3method(is_fill,default)
+S3method(is_fill,htmlwidget)
+S3method(is_fillable,default)
+S3method(is_fillable,htmlwidget)
 S3method(print,bslib_fragment)
 S3method(print,bslib_page)
 export(accordion)
@@ -11,6 +15,9 @@ export(accordion_panel_remove)
 export(accordion_panel_set)
 export(accordion_panel_update)
 export(as.card_item)
+export(as_fill)
+export(as_fill_carrier)
+export(as_fillable)
 export(bootstrap)
 export(bootstrap_sass)
 export(bootswatch_themes)
@@ -63,6 +70,9 @@ export(font_google)
 export(font_link)
 export(is.card_item)
 export(is_bs_theme)
+export(is_fill)
+export(is_fill_carrier)
+export(is_fillable)
 export(layout_column_wrap)
 export(layout_sidebar)
 export(nav)
@@ -99,6 +109,7 @@ export(sidebar_close)
 export(sidebar_open)
 export(theme_bootswatch)
 export(theme_version)
+export(undo_fill)
 export(value_box)
 export(version_default)
 export(versions)
diff --git a/NEWS.md b/NEWS.md
index 5f0b51960..beebe33cb 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -25,6 +25,8 @@ Although `{bslib}` is still maturing, and will continue to receiving new UI feat
 * `page_fillable()` (aka, `page_fill()`) is now considered a `fillable` container, meaning that `fill` items like `card()`, `layout_column_wrap()`, and `layout_sidebar()` now grow/shrink to fit the window's height when they appear as a direct child of `page_fillable()`. (#479)
 * `page_navbar()` and `page_fillable()` gain `fill_mobile` arguments to control whether the page should grow/shrink to fit the viewport on mobile. (#479)
 * `card()`, `value_box()`, and `card_image()` gain `max_height` and `fill` arguments. (#498)
+* Added new `as_fill()`, `as_fillable()`, `as_fill_carrier()`, `is_fill()`, and `is_fillable()` for testing and coercing potential to fill. (#498)
+
 
 ## Deprecations
 
diff --git a/R/fill.R b/R/fill.R
new file mode 100644
index 000000000..7f2be48b1
--- /dev/null
+++ b/R/fill.R
@@ -0,0 +1,164 @@
+#' Test and/or coerce fill behavior
+#'
+#' @description Filling layouts in bslib are built on the foundation of fillable
+#' containers and fill items (fill carriers are both fillable and
+#' fill). This is why most bslib components (e.g., [card()], [card_body()],
+#' [layout_sidebar()]) possess both `fillable` and `fill` arguments (to control
+#' their fill behavior). However, sometimes it's useful to add, remove, and/or
+#' test fillable/fill properties on arbitrary [htmltools::tag()], which these
+#' functions are designed to do.
+#'
+#' @references <https://rstudio.github.io/bslib/articles/filling.html>
+#'
+#' @details Although `as_fill()`, `as_fillable()`, and `as_fill_carrier()`
+#' can work with non-tag objects that have a [as.tags] method (e.g., htmlwidgets),
+#' they return the "tagified" version of that object
+#'
+#' @return
+#'   * For `as_fill()`, `as_fillable()`, and `as_fill_carrier()`: the _tagified_
+#'     version `x`, with relevant tags modified to possess the relevant fill
+#'     properties.
+#'   * For `is_fill()`, `is_fillable()`, and `is_fill_carrier()`: a logical vector,
+#'     with length matching the number of top-level tags that possess the relevant
+#'     fill properties.
+#'
+#' @param x a [htmltools::tag()].
+#' @param ... currently ignored.
+#' @param min_height,max_height Any valid [CSS unit][htmltools::validateCssUnit]
+#'   (e.g., `150`).
+#' @param gap Any valid [CSS unit][htmltools::validateCssUnit].
+#' @param class A character vector of class names to add to the tag.
+#' @param style A character vector of CSS properties to add to the tag.
+#' @param css_selector A character string containing a CSS selector for
+#'   targeting particular (inner) tag(s) of interest. For more details on what
+#'   selector(s) are supported, see [tagAppendAttributes()].
+#' @export
+as_fill_carrier <- function(x, ..., min_height = NULL, max_height = NULL, gap = NULL, class = NULL, style = NULL, css_selector = NULL) {
+
+  rlang::check_dots_empty()
+
+  x <- as_fillable(
+    x, min_height = min_height,
+    max_height = max_height,
+    gap = gap,
+    class = class,
+    style = style,
+    css_selector = css_selector
+  )
+
+  bindFillRole(x, item = TRUE, .cssSelector = css_selector)
+}
+
+
+#' @rdname as_fill_carrier
+#' @export
+as_fillable <- function(x, ..., min_height = NULL, max_height = NULL, gap = NULL, class = NULL, style = NULL, css_selector = NULL) {
+
+  rlang::check_dots_empty()
+
+  x <- bindFillRole(x, container = TRUE, .cssSelector = css_selector)
+
+  tagAppendAttributes(
+    x, .cssSelector = css_selector,
+    style = css(
+      min_height = validateCssUnit(min_height),
+      max_height = validateCssUnit(max_height),
+      gap = validateCssUnit(gap)
+    ),
+    class = class,
+    style = style
+  )
+}
+
+#' @rdname as_fill_carrier
+#' @export
+as_fill <- function(x, ..., min_height = NULL, max_height = NULL, class = NULL, style = NULL, css_selector = NULL) {
+
+  rlang::check_dots_empty()
+
+  x <- bindFillRole(x, item = TRUE, .cssSelector = css_selector)
+
+  tagAppendAttributes(
+    x, .cssSelector = css_selector,
+    style = css(
+      min_height = validateCssUnit(min_height),
+      max_height = validateCssUnit(max_height)
+    ),
+    class = class,
+    style = style
+  )
+}
+
+
+#' @rdname as_fill_carrier
+#' @export
+undo_fill <- function(x) {
+  bindFillRole(
+    x, item = FALSE, container = FALSE,
+    overwrite = TRUE
+  )
+}
+
+
+#' @rdname as_fill_carrier
+#' @export
+is_fill_carrier <- function(x) {
+  classes <- paste0("html-fill-", c("container", "item"))
+  renders_to_tag_class(x, classes)
+}
+
+#' @rdname as_fill_carrier
+#' @export
+is_fillable <- function(x) {
+  UseMethod("is_fillable")
+}
+
+#' @export
+is_fillable.htmlwidget <- function(x) {
+  # won't actually work until (htmltools#334) gets fixed
+  renders_to_tag_class(x, "html-fill-container", ".html-widget")
+}
+
+#' @export
+is_fillable.default <- function(x) {
+  renders_to_tag_class(x, "html-fill-container")
+}
+
+#' @rdname as_fill_carrier
+#' @export
+is_fill <- function(x) {
+  UseMethod("is_fill")
+}
+
+#' @export
+is_fill.htmlwidget <- function(x) {
+  # won't actually work until (htmltools#334) gets fixed
+  renders_to_tag_class(x, "html-fill-item", ".html-widget")
+}
+
+#' @export
+is_fill.default <- function(x) {
+  renders_to_tag_class(x, "html-fill-item")
+}
+
+
+renders_to_tag_class <- function(x, class, selector = NULL) {
+  x <- try(as.tags(x), silent = TRUE)
+  if (inherits(x, "try-error")) {
+    return(FALSE)
+  }
+  xq <- tagQuery(x)
+  if (length(selector)) {
+    xq <- xq$find(selector)
+  }
+  vapply(
+    xq$selectedTags(),
+    function(y) tagQuery(y)$hasClass(class),
+    logical(1),
+    USE.NAMES = FALSE
+  )
+}
+
+is_tag <- function(x) {
+  inherits(x, "shiny.tag")
+}
diff --git a/R/sysdata.rda b/R/sysdata.rda
index 24331eafe..5320aaa4a 100644
Binary files a/R/sysdata.rda and b/R/sysdata.rda differ
diff --git a/_pkgdown.yml b/_pkgdown.yml
index 6ef775561..60c69e676 100644
--- a/_pkgdown.yml
+++ b/_pkgdown.yml
@@ -105,6 +105,11 @@ reference:
 - title: Page layouts
   contents:
     - page
+- title: Fill items and fillable containers
+  description: |
+    Create and test for fill items and fillable containers
+  contents:
+    - as_fill_carrier
 - title: Dynamic theming
   description: |
     Create dynamically themable HTML widgets.
diff --git a/man/as_fill_carrier.Rd b/man/as_fill_carrier.Rd
new file mode 100644
index 000000000..19fe6456e
--- /dev/null
+++ b/man/as_fill_carrier.Rd
@@ -0,0 +1,97 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/fill.R
+\name{as_fill_carrier}
+\alias{as_fill_carrier}
+\alias{as_fillable}
+\alias{as_fill}
+\alias{undo_fill}
+\alias{is_fill_carrier}
+\alias{is_fillable}
+\alias{is_fill}
+\title{Test and/or coerce fill behavior}
+\usage{
+as_fill_carrier(
+  x,
+  ...,
+  min_height = NULL,
+  max_height = NULL,
+  gap = NULL,
+  class = NULL,
+  style = NULL,
+  css_selector = NULL
+)
+
+as_fillable(
+  x,
+  ...,
+  min_height = NULL,
+  max_height = NULL,
+  gap = NULL,
+  class = NULL,
+  style = NULL,
+  css_selector = NULL
+)
+
+as_fill(
+  x,
+  ...,
+  min_height = NULL,
+  max_height = NULL,
+  class = NULL,
+  style = NULL,
+  css_selector = NULL
+)
+
+undo_fill(x)
+
+is_fill_carrier(x)
+
+is_fillable(x)
+
+is_fill(x)
+}
+\arguments{
+\item{x}{a \code{\link[htmltools:builder]{htmltools::tag()}}.}
+
+\item{...}{currently ignored.}
+
+\item{min_height, max_height}{Any valid \link[htmltools:validateCssUnit]{CSS unit}
+(e.g., \code{150}).}
+
+\item{gap}{Any valid \link[htmltools:validateCssUnit]{CSS unit}.}
+
+\item{class}{A character vector of class names to add to the tag.}
+
+\item{style}{A character vector of CSS properties to add to the tag.}
+
+\item{css_selector}{A character string containing a CSS selector for
+targeting particular (inner) tag(s) of interest. For more details on what
+selector(s) are supported, see \code{\link[=tagAppendAttributes]{tagAppendAttributes()}}.}
+}
+\value{
+\itemize{
+\item For \code{as_fill()}, \code{as_fillable()}, and \code{as_fill_carrier()}: the \emph{tagified}
+version \code{x}, with relevant tags modified to possess the relevant fill
+properties.
+\item For \code{is_fill()}, \code{is_fillable()}, and \code{is_fill_carrier()}: a logical vector,
+with length matching the number of top-level tags that possess the relevant
+fill properties.
+}
+}
+\description{
+Filling layouts in bslib are built on the foundation of fillable
+containers and fill items (fill carriers are both fillable and
+fill). This is why most bslib components (e.g., \code{\link[=card]{card()}}, \code{\link[=card_body]{card_body()}},
+\code{\link[=layout_sidebar]{layout_sidebar()}}) possess both \code{fillable} and \code{fill} arguments (to control
+their fill behavior). However, sometimes it's useful to add, remove, and/or
+test fillable/fill properties on arbitrary \code{\link[htmltools:builder]{htmltools::tag()}}, which these
+functions are designed to do.
+}
+\details{
+Although \code{as_fill()}, \code{as_fillable()}, and \code{as_fill_carrier()}
+can work with non-tag objects that have a \link{as.tags} method (e.g., htmlwidgets),
+they return the "tagified" version of that object
+}
+\references{
+\url{https://rstudio.github.io/bslib/articles/filling.html}
+}