diff --git a/Kernel/FileSystem/ProcFS.cpp b/Kernel/FileSystem/ProcFS.cpp
index 37da0d35a9..12250da5c5 100644
--- a/Kernel/FileSystem/ProcFS.cpp
+++ b/Kernel/FileSystem/ProcFS.cpp
@@ -822,9 +822,9 @@ Optional<KBuffer> procfs$all(InodeIdentifier)
         }
 
         process_object.add("pid", process.pid().value());
-        process_object.add("pgid", process.tty() ? process.tty()->pgid() : 0);
-        process_object.add("pgp", process.pgid());
-        process_object.add("sid", process.sid());
+        process_object.add("pgid", process.tty() ? process.tty()->pgid().value() : 0);
+        process_object.add("pgp", process.pgid().value());
+        process_object.add("sid", process.sid().value());
         process_object.add("uid", process.uid());
         process_object.add("gid", process.gid());
         process_object.add("ppid", process.ppid().value());
diff --git a/Kernel/Process.cpp b/Kernel/Process.cpp
index e281bfc364..4a6eefb07c 100644
--- a/Kernel/Process.cpp
+++ b/Kernel/Process.cpp
@@ -100,7 +100,8 @@ ProcessID Process::allocate_pid()
     // Overflow is UB, and negative PIDs wreck havoc.
     // TODO: Handle PID overflow
     // For example: Use an Atomic<u32>, mask the most significant bit,
-    // retry if PID is already taken as a PID, taken as a TID, or zero.
+    // retry if PID is already taken as a PID, taken as a TID,
+    // takes as a PGID, taken as a SID, or zero.
     return next_pid.fetch_add(1, AK::MemoryOrder::memory_order_acq_rel);
 }
 
diff --git a/Kernel/Process.h b/Kernel/Process.h
index d024e629ab..f06af2732b 100644
--- a/Kernel/Process.h
+++ b/Kernel/Process.h
@@ -153,11 +153,14 @@ public:
     const PageDirectory& page_directory() const { return *m_page_directory; }
 
     static RefPtr<Process> from_pid(ProcessID);
+    static SessionID get_sid_from_pgid(ProcessGroupID pgid);
 
     const String& name() const { return m_name; }
     ProcessID pid() const { return m_pid; }
-    pid_t sid() const { return m_sid; }
-    pid_t pgid() const { return m_pgid; }
+    SessionID sid() const { return m_sid; }
+    bool is_session_leader() const { return m_sid.value() == m_pid.value(); }
+    ProcessGroupID pgid() const { return m_pgid; }
+    bool is_group_leader() const { return m_pgid.value() == m_pid.value(); }
     const FixedArray<gid_t>& extra_gids() const { return m_extra_gids; }
     uid_t euid() const { return m_euid; }
     gid_t egid() const { return m_egid; }
@@ -179,7 +182,7 @@ public:
     template<typename Callback>
     static void for_each(Callback);
     template<typename Callback>
-    static void for_each_in_pgrp(pid_t, Callback);
+    static void for_each_in_pgrp(ProcessGroupID, Callback);
     template<typename Callback>
     void for_each_child(Callback);
     template<typename Callback>
@@ -617,8 +620,8 @@ private:
     String m_name;
 
     ProcessID m_pid { 0 };
-    pid_t m_sid { 0 };
-    pid_t m_pgid { 0 };
+    SessionID m_sid { 0 };
+    ProcessGroupID m_pgid { 0 };
 
     uid_t m_euid { 0 };
     gid_t m_egid { 0 };
@@ -772,7 +775,7 @@ inline void Process::for_each_thread(Callback callback) const
 }
 
 template<typename Callback>
-inline void Process::for_each_in_pgrp(pid_t pgid, Callback callback)
+inline void Process::for_each_in_pgrp(ProcessGroupID pgid, Callback callback)
 {
     ASSERT_INTERRUPTS_DISABLED();
     ScopedSpinLock lock(g_processes_lock);
diff --git a/Kernel/Syscalls/kill.cpp b/Kernel/Syscalls/kill.cpp
index 2ac7dd712f..6331280f3b 100644
--- a/Kernel/Syscalls/kill.cpp
+++ b/Kernel/Syscalls/kill.cpp
@@ -52,7 +52,8 @@ KResult Process::do_killpg(pid_t pgrp, int signal)
     // Send the signal to all processes in the given group.
     if (pgrp == 0) {
         // Send the signal to our own pgrp.
-        pgrp = pgid();
+        // FIXME: PIF/PGID INCOMPLETE
+        pgrp = pgid().value();
     }
 
     bool group_was_empty = true;
diff --git a/Kernel/Syscalls/setpgid.cpp b/Kernel/Syscalls/setpgid.cpp
index 5561131fbb..457f44e7ed 100644
--- a/Kernel/Syscalls/setpgid.cpp
+++ b/Kernel/Syscalls/setpgid.cpp
@@ -33,14 +33,14 @@ pid_t Process::sys$getsid(pid_t pid)
 {
     REQUIRE_PROMISE(proc);
     if (pid == 0)
-        return m_sid;
+        return m_sid.value();
     ScopedSpinLock lock(g_processes_lock);
     auto process = Process::from_pid(pid);
     if (!process)
         return -ESRCH;
     if (m_sid != process->m_sid)
         return -EPERM;
-    return process->m_sid;
+    return process->m_sid.value();
 }
 
 pid_t Process::sys$setsid()
@@ -48,44 +48,49 @@ pid_t Process::sys$setsid()
     REQUIRE_PROMISE(proc);
     InterruptDisabler disabler;
     bool found_process_with_same_pgid_as_my_pid = false;
-    // FIXME: PID/PGID ISSUE?
     Process::for_each_in_pgrp(pid().value(), [&](auto&) {
         found_process_with_same_pgid_as_my_pid = true;
         return IterationDecision::Break;
     });
     if (found_process_with_same_pgid_as_my_pid)
         return -EPERM;
+    // Create a new Session and a new ProcessGroup.
     m_sid = m_pid.value();
     m_pgid = m_pid.value();
     m_tty = nullptr;
-    return m_sid;
+    return m_sid.value();
 }
 
 pid_t Process::sys$getpgid(pid_t pid)
 {
     REQUIRE_PROMISE(proc);
     if (pid == 0)
-        return m_pgid;
+        return m_pgid.value();
     ScopedSpinLock lock(g_processes_lock); // FIXME: Use a ProcessHandle
     auto process = Process::from_pid(pid);
     if (!process)
         return -ESRCH;
-    return process->m_pgid;
+    return process->m_pgid.value();
 }
 
 pid_t Process::sys$getpgrp()
 {
     REQUIRE_PROMISE(stdio);
-    return m_pgid;
+    return m_pgid.value();
 }
 
-static pid_t get_sid_from_pgid(pid_t pgid)
+SessionID Process::get_sid_from_pgid(ProcessGroupID pgid)
 {
+    // FIXME: This xor sys$setsid() uses the wrong locking mechanism.
     ScopedSpinLock lock(g_processes_lock);
-    auto group_leader = Process::from_pid(pgid);
-    if (!group_leader)
-        return -1;
-    return group_leader->sid();
+
+    SessionID sid { -1 };
+    Process::for_each_in_pgrp(pgid, [&](auto& process) {
+        sid = process.sid();
+        return IterationDecision::Break;
+    });
+
+    return sid;
 }
 
 int Process::sys$setpgid(pid_t specified_pid, pid_t specified_pgid)
@@ -105,7 +110,7 @@ int Process::sys$setpgid(pid_t specified_pid, pid_t specified_pgid)
         // of the calling process or of a child process of the calling process.
         return -ESRCH;
     }
-    if (process->pid() == process->sid()) {
+    if (process->is_session_leader()) {
         // The process indicated by the pid argument is a session leader.
         return -EPERM;
     }
@@ -116,10 +121,9 @@ int Process::sys$setpgid(pid_t specified_pid, pid_t specified_pgid)
         return -EPERM;
     }
 
-    // FIXME: PID/PGID INCOMPLETE
-    pid_t new_pgid = specified_pgid ? specified_pgid : process->m_pid.value();
-    pid_t current_sid = get_sid_from_pgid(process->m_pgid);
-    pid_t new_sid = get_sid_from_pgid(new_pgid);
+    ProcessGroupID new_pgid = specified_pgid ? ProcessGroupID(specified_pgid) : process->m_pid.value();
+    SessionID current_sid = sid();
+    SessionID new_sid = get_sid_from_pgid(new_pgid);
     if (current_sid != new_sid) {
         // Can't move a process between sessions.
         return -EPERM;
diff --git a/Kernel/TTY/TTY.cpp b/Kernel/TTY/TTY.cpp
index e3b655d24f..28e781927e 100644
--- a/Kernel/TTY/TTY.cpp
+++ b/Kernel/TTY/TTY.cpp
@@ -159,6 +159,7 @@ void TTY::emit(u8 ch)
                 if (auto parent = Process::from_pid(m_process->ppid()))
                     (void)parent->send_signal(SIGCHLD, m_process);
             }
+            // TODO: Else send it to the session leader maybe?
             return;
         }
     }
@@ -253,7 +254,7 @@ void TTY::generate_signal(int signal)
         return;
     if (should_flush_on_signal())
         flush_input();
-    dbg() << tty_name() << ": Send signal " << signal << " to everyone in pgrp " << pgid();
+    dbg() << tty_name() << ": Send signal " << signal << " to everyone in pgrp " << pgid().value();
     InterruptDisabler disabler; // FIXME: Iterate over a set of process handles instead?
     Process::for_each_in_pgrp(pgid(), [&](auto& process) {
         dbg() << tty_name() << ": Send signal " << signal << " to " << process;
@@ -291,7 +292,6 @@ int TTY::ioctl(FileDescription&, unsigned request, FlatPtr arg)
 {
     REQUIRE_PROMISE(tty);
     auto& current_process = *Process::current();
-    pid_t pgid;
     termios* user_termios;
     winsize* user_winsize;
 
@@ -304,23 +304,21 @@ int TTY::ioctl(FileDescription&, unsigned request, FlatPtr arg)
 #endif
     switch (request) {
     case TIOCGPGRP:
-        return this->pgid();
-    case TIOCSPGRP:
-        pgid = static_cast<pid_t>(arg);
+        return this->pgid().value();
+    case TIOCSPGRP: {
+        ProcessGroupID pgid = static_cast<pid_t>(arg);
         if (pgid <= 0)
             return -EINVAL;
-        {
-            InterruptDisabler disabler;
-            auto process = Process::from_pid(pgid);
-            if (!process)
-                return -EPERM;
-            if (pgid != process->pgid())
-                return -EPERM;
-            if (current_process.sid() != process->sid())
-                return -EPERM;
-            m_process = process->make_weak_ptr();
-        }
+        InterruptDisabler disabler;
+        auto process = Process::from_pid(pgid.value());
+        SessionID new_sid = process ? process->sid() : Process::get_sid_from_pgid(pgid);
+        if (!new_sid || new_sid != current_process.sid())
+            return -EPERM;
+        if (process && pgid != process->pgid())
+            return -EPERM;
+        m_process = process ? process->make_weak_ptr() : WeakPtr<Process>();
         return 0;
+    }
     case TCGETS: {
         user_termios = reinterpret_cast<termios*>(arg);
         if (!current_process.validate_write(user_termios, sizeof(termios)))
@@ -395,7 +393,7 @@ void TTY::hang_up()
     generate_signal(SIGHUP);
 }
 
-pid_t TTY::pgid() const
+ProcessGroupID TTY::pgid() const
 {
     return m_process ? m_process->pgid() : 0;
 }
diff --git a/Kernel/TTY/TTY.h b/Kernel/TTY/TTY.h
index cbaf23e9e6..e8febbd079 100644
--- a/Kernel/TTY/TTY.h
+++ b/Kernel/TTY/TTY.h
@@ -50,7 +50,7 @@ public:
     unsigned short rows() const { return m_rows; }
     unsigned short columns() const { return m_columns; }
 
-    pid_t pgid() const;
+    ProcessGroupID pgid() const;
 
     void set_termios(const termios&);
     bool should_generate_signals() const { return m_termios.c_lflag & ISIG; }