diff --git a/NEWS.md b/NEWS.md
index 54d6a9d3..7ab4a4b0 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -4,12 +4,22 @@
 
 ## Updates of Existing Functions
 
+* Error messaging throughout the package have been updated from `rlang::abort()` to `cli::cli_abort()`. As a part of the update, each of the `assert_*()` functions have new arguments `assert_*(message, arg_name, call, class).` (#367)
+
+* Warning messaging has also been updated to use `{cli}` messaging.
+
 ## Breaking Changes
 
 * `renv` and related files have been removed. (#360)
 
 * No longer exporting `is_named()` function. (#401)
 
+* As a part of the error messaging update, the following changes were made.
+
+    - The `assert_s3_class(class)` argument has been renamed to `assert_s3_class(cls)`. (#367)
+
+    - Functions `arg_name()`, `enumerate()`, `what_is_it()`, and `friendly_type_of()` have been deprecated and a warning is returned to any developer using these functions. As these are developer functions (as opposed to functions for typical admiral users), we will use a short deprecation cycle.
+
 ## Documentation
 
 - The "Release Strategy" vignette was updated with respect to the new branching
diff --git a/R/compat_friendly_type.R b/R/compat_friendly_type.R
index 01ef4ad4..f84345c1 100644
--- a/R/compat_friendly_type.R
+++ b/R/compat_friendly_type.R
@@ -17,6 +17,13 @@
 #' @keywords dev_utility
 #' @family dev_utility
 friendly_type_of <- function(x, value = TRUE, length = FALSE) { # nolint
+  lifecycle::deprecate_warn(
+    when = "1.1.0",
+    what = "admiraldev::friendly_type_of()",
+    details = "This function was primarily used in error messaging, and can be replaced
+               with 'cli' functionality: `cli::cli_abort('{.obj_type_friendly {letters}}')`."
+  )
+
   if (rlang::is_missing(x)) {
     return("absent")
   }
diff --git a/R/dev_utilities.R b/R/dev_utilities.R
index fb540c79..6f7f31b1 100644
--- a/R/dev_utilities.R
+++ b/R/dev_utilities.R
@@ -44,6 +44,13 @@ convert_dtm_to_dtc <- function(dtm) {
 #'
 #' @export
 arg_name <- function(expr) { # nolint
+  lifecycle::deprecate_warn(
+    when = "1.1.0",
+    what = "admiraldev::arg_name()",
+    details = "This function was primarily used in error messaging, and can be
+               replaced with `assert_*(x, arg_name = rlang::caller_arg(x))`"
+  )
+
   if (length(expr) == 1L && is.symbol(expr)) {
     deparse(expr)
   } else if (length(expr) == 2L &&
diff --git a/R/quote.R b/R/quote.R
index ae0a5b99..03342350 100644
--- a/R/quote.R
+++ b/R/quote.R
@@ -20,6 +20,13 @@
 #'
 #' @export
 enumerate <- function(x, quote_fun = backquote, conjunction = "and") {
+  lifecycle::deprecate_warn(
+    when = "1.1.0",
+    what = "admiraldev::enumerate()",
+    details = "This function was primarily used in error messaging, and can be
+               replaced with 'cli' functionality: `cli::cli_abort('{.val {letters[1:3]}}')`"
+  )
+
   if (is.null(quote_fun)) {
     quote_fun <- function(x) x
   }
diff --git a/R/warnings.R b/R/warnings.R
index dc0cb069..d0250bc7 100644
--- a/R/warnings.R
+++ b/R/warnings.R
@@ -24,15 +24,12 @@
 #' warn_if_vars_exist(dm, "ARM")
 warn_if_vars_exist <- function(dataset, vars) {
   existing_vars <- vars[vars %in% colnames(dataset)]
-  if (length(existing_vars) == 1L) {
-    msg <- paste("Variable", backquote(existing_vars), "already exists in the dataset")
-    warn(msg)
-  } else if (length(existing_vars) > 1L) {
-    msg <- paste("Variables", enumerate(existing_vars), "already exist in the dataset")
-    warn(msg)
-  } else {
-    invisible(NULL)
+  if (length(existing_vars) >= 1L) {
+    cli::cli_warn("{cli::qty(length(existing_vars))}Variable{?s} {.val {existing_vars}}
+                   already {?exists/exist} in the dataset.")
   }
+
+  invisible(NULL)
 }
 
 #' Warn If a Vector Contains Unknown Datetime Format
@@ -74,7 +71,7 @@ warn_if_invalid_dtc <- function(dtc, is_valid = is_valid_dtc(dtc)) {
       "Missing parts in the middle must be represented by a dash, e.g., 2003---15.",
       sep = "\n"
     )
-    warn(paste(main_msg, tbl, info, sep = "\n"))
+    cli::cli_warn(c(main_msg, "*" = tbl, "*" = info))
   }
 }
 
diff --git a/R/what.R b/R/what.R
index 575f6652..92c78068 100644
--- a/R/what.R
+++ b/R/what.R
@@ -18,6 +18,13 @@
 #' what_is_it(1:10)
 #' what_is_it(mtcars)
 what_is_it <- function(x) {
+  lifecycle::deprecate_warn(
+    when = "1.1.0",
+    what = "admiraldev::what_is_it()",
+    details = "This function was primarily used in error messaging, and can be replaced
+               with 'cli' functionality: `cli::cli_abort('{.obj_type_friendly {letters}}')`."
+  )
+
   if (is.null(x)) {
     "`NULL`"
   } else if (is.factor(x)) {
diff --git a/inst/WORDLIST b/inst/WORDLIST
index f045a594..cfaea46f 100644
--- a/inst/WORDLIST
+++ b/inst/WORDLIST
@@ -52,6 +52,7 @@ admiraltemplate
 admiralxxx
 advs
 anonymized
+cli
 cmd
 codebase
 codeowner
diff --git a/tests/testthat/_snaps/warnings.md b/tests/testthat/_snaps/warnings.md
new file mode 100644
index 00000000..db57d56d
--- /dev/null
+++ b/tests/testthat/_snaps/warnings.md
@@ -0,0 +1,34 @@
+# warn_if_vars_exist Test 1: warning if a variable already exists in the input dataset
+
+    Code
+      warn_if_vars_exist(dm, "AGE")
+    Condition
+      Warning:
+      Variable "AGE" already exists in the dataset.
+
+---
+
+    Code
+      warn_if_vars_exist(dm, c("AGE", "AGEU", "ARM"))
+    Condition
+      Warning:
+      Variables "AGE", "AGEU", and "ARM" already exist in the dataset.
+
+---
+
+    Code
+      warn_if_vars_exist(dm, c("AAGE", "AGEU", "ARM"))
+    Condition
+      Warning:
+      Variables "AGEU" and "ARM" already exist in the dataset.
+
+# warn_if_invalid_dtc Test 2: Warning if vector contains unknown datetime format
+
+    Code
+      warn_if_invalid_dtc(dtc = "20210406T12:30:30")
+    Condition
+      Warning:
+      Dataset contains incorrect datetime format: --DTC may be incorrectly imputed on row(s)
+      * Row 1 : --DTC = 20210406T12:30:30
+      * ISO representations of the form YYYY-MM-DDThh:mm:ss.ddd are expected, e.g., 2003-12-15T13:15:17.123. Missing parts at the end can be omitted. Missing parts in the middle must be represented by a dash, e.g., 2003---15.
+
diff --git a/tests/testthat/test-warnings.R b/tests/testthat/test-warnings.R
index a0288440..204110fb 100644
--- a/tests/testthat/test-warnings.R
+++ b/tests/testthat/test-warnings.R
@@ -3,17 +3,14 @@
 test_that("warn_if_vars_exist Test 1: warning if a variable already exists in the input dataset", {
   dm <- pharmaversesdtm::dm
 
-  expect_warning(
-    warn_if_vars_exist(dm, "AGE"),
-    "Variable `AGE` already exists in the dataset"
+  expect_snapshot(
+    warn_if_vars_exist(dm, "AGE")
   )
-  expect_warning(
-    warn_if_vars_exist(dm, c("AGE", "AGEU", "ARM")),
-    "Variables `AGE`, `AGEU` and `ARM` already exist in the dataset"
+  expect_snapshot(
+    warn_if_vars_exist(dm, c("AGE", "AGEU", "ARM"))
   )
-  expect_warning(
-    warn_if_vars_exist(dm, c("AAGE", "AGEU", "ARM")),
-    "Variables `AGEU` and `ARM` already exist in the dataset"
+  expect_snapshot(
+    warn_if_vars_exist(dm, c("AAGE", "AGEU", "ARM"))
   )
   expect_warning(
     warn_if_vars_exist(dm, "AAGE"),
@@ -24,9 +21,8 @@ test_that("warn_if_vars_exist Test 1: warning if a variable already exists in th
 # warn_if_invalid_dtc ----
 ## Test 2: Warning if vector contains unknown datetime format ----
 test_that("warn_if_invalid_dtc Test 2: Warning if vector contains unknown datetime format", {
-  expect_warning(
-    warn_if_invalid_dtc(dtc = "20210406T12:30:30"),
-    "Dataset contains incorrect datetime format:"
+  expect_snapshot(
+    warn_if_invalid_dtc(dtc = "20210406T12:30:30")
   )
 })